diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index 8281215..e4b9992 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -2082,4 +2082,20 @@ add_executable(step346_test tests/step346_test.cpp)
 target_include_directories(step346_test PRIVATE src)
 target_link_libraries(step346_test PRIVATE nlohmann_json::nlohmann_json)
 
+add_executable(step347_test tests/step347_test.cpp)
+target_include_directories(step347_test PRIVATE src)
+target_link_libraries(step347_test PRIVATE nlohmann_json::nlohmann_json)
+
+add_executable(step348_test tests/step348_test.cpp)
+target_include_directories(step348_test PRIVATE src)
+target_link_libraries(step348_test PRIVATE nlohmann_json::nlohmann_json)
+
+add_executable(step349_test tests/step349_test.cpp)
+target_include_directories(step349_test PRIVATE src)
+target_link_libraries(step349_test PRIVATE nlohmann_json::nlohmann_json)
+
+add_executable(step350_test tests/step350_test.cpp)
+target_include_directories(step350_test PRIVATE src)
+target_link_libraries(step350_test PRIVATE nlohmann_json::nlohmann_json)
+
 # Step 12: Dear ImGui shell scaffolding created (main.cpp exists but not built due to dependencies)
diff --git a/editor/src/ThemeData.h b/editor/src/ThemeData.h
index b77e1fd..373fb4c 100644
--- a/editor/src/ThemeData.h
+++ b/editor/src/ThemeData.h
@@ -71,17 +71,39 @@ struct WhetstoneTheme {
     nlohmann::json toJson() const {
         nlohmann::json j;
         j["name"] = name;
+        // Backgrounds
         j["bg"] = bg.toHex(); j["bgAlt"] = bgAlt.toHex();
         j["bgPanel"] = bgPanel.toHex(); j["bgPopup"] = bgPopup.toHex();
+        // Text
         j["text"] = text.toHex(); j["textDim"] = textDim.toHex();
-        j["accent"] = accent.toHex();
+        j["textAccent"] = textAccent.toHex();
+        j["textError"] = textError.toHex(); j["textWarning"] = textWarning.toHex();
+        j["textSuccess"] = textSuccess.toHex();
+        // Borders
+        j["border"] = border.toHex(); j["borderFocused"] = borderFocused.toHex();
+        // Accent
+        j["accent"] = accent.toHex(); j["accentHover"] = accentHover.toHex();
+        j["accentActive"] = accentActive.toHex();
+        // Selection
+        j["selectionBg"] = selectionBg.toHex(); j["selectionText"] = selectionText.toHex();
+        // Scrollbar
+        j["scrollbar"] = scrollbar.toHex(); j["scrollbarHover"] = scrollbarHover.toHex();
+        // Diagnostics
         j["diagError"] = diagError.toHex(); j["diagWarning"] = diagWarning.toHex();
+        j["diagInfo"] = diagInfo.toHex(); j["diagHint"] = diagHint.toHex();
+        // Syntax
         j["syntaxKeyword"] = syntaxKeyword.toHex(); j["syntaxString"] = syntaxString.toHex();
         j["syntaxNumber"] = syntaxNumber.toHex(); j["syntaxComment"] = syntaxComment.toHex();
+        j["syntaxFunction"] = syntaxFunction.toHex(); j["syntaxType"] = syntaxType.toHex();
+        j["syntaxOperator"] = syntaxOperator.toHex(); j["syntaxAnnotation"] = syntaxAnnotation.toHex();
+        // Fonts
         j["fontSizePrimary"] = fontSizePrimary;
         j["fontSizeUI"] = fontSizeUI;
+        j["fontSizeSmall"] = fontSizeSmall;
         return j;
     }
+
+    static WhetstoneTheme fromJson(const nlohmann::json& j);
 };
 
 inline WhetstoneTheme getDefaultDarkTheme() {
@@ -183,3 +205,38 @@ inline WhetstoneTheme getDefaultLightTheme() {
 
     return t;
 }
+
+inline WhetstoneTheme WhetstoneTheme::fromJson(const nlohmann::json& j) {
+    WhetstoneTheme t;
+    auto hex = [&](const std::string& key, Color4 fallback) -> Color4 {
+        if (j.contains(key) && j[key].is_string()) return Color4::fromHex(j[key].get<std::string>());
+        return fallback;
+    };
+    auto num = [&](const std::string& key, float fallback) -> float {
+        if (j.contains(key) && j[key].is_number()) return j[key].get<float>();
+        return fallback;
+    };
+    t.name = j.value("name", "Custom");
+    auto def = getDefaultDarkTheme();
+    t.bg = hex("bg", def.bg); t.bgAlt = hex("bgAlt", def.bgAlt);
+    t.bgPanel = hex("bgPanel", def.bgPanel); t.bgPopup = hex("bgPopup", def.bgPopup);
+    t.text = hex("text", def.text); t.textDim = hex("textDim", def.textDim);
+    t.textAccent = hex("textAccent", def.textAccent);
+    t.textError = hex("textError", def.textError); t.textWarning = hex("textWarning", def.textWarning);
+    t.textSuccess = hex("textSuccess", def.textSuccess);
+    t.border = hex("border", def.border); t.borderFocused = hex("borderFocused", def.borderFocused);
+    t.accent = hex("accent", def.accent); t.accentHover = hex("accentHover", def.accentHover);
+    t.accentActive = hex("accentActive", def.accentActive);
+    t.selectionBg = hex("selectionBg", def.selectionBg); t.selectionText = hex("selectionText", def.selectionText);
+    t.scrollbar = hex("scrollbar", def.scrollbar); t.scrollbarHover = hex("scrollbarHover", def.scrollbarHover);
+    t.diagError = hex("diagError", def.diagError); t.diagWarning = hex("diagWarning", def.diagWarning);
+    t.diagInfo = hex("diagInfo", def.diagInfo); t.diagHint = hex("diagHint", def.diagHint);
+    t.syntaxKeyword = hex("syntaxKeyword", def.syntaxKeyword); t.syntaxString = hex("syntaxString", def.syntaxString);
+    t.syntaxNumber = hex("syntaxNumber", def.syntaxNumber); t.syntaxComment = hex("syntaxComment", def.syntaxComment);
+    t.syntaxFunction = hex("syntaxFunction", def.syntaxFunction); t.syntaxType = hex("syntaxType", def.syntaxType);
+    t.syntaxOperator = hex("syntaxOperator", def.syntaxOperator); t.syntaxAnnotation = hex("syntaxAnnotation", def.syntaxAnnotation);
+    t.fontSizePrimary = num("fontSizePrimary", def.fontSizePrimary);
+    t.fontSizeUI = num("fontSizeUI", def.fontSizeUI);
+    t.fontSizeSmall = num("fontSizeSmall", def.fontSizeSmall);
+    return t;
+}
diff --git a/editor/tests/step347_test.cpp b/editor/tests/step347_test.cpp
new file mode 100644
index 0000000..c849aaf
--- /dev/null
+++ b/editor/tests/step347_test.cpp
@@ -0,0 +1,190 @@
+// Step 347: Theme System — Full Theme Roundtrip + Validation (12 tests)
+// Tests theme JSON serialization/deserialization, color completeness, contrast
+
+#include <cassert>
+#include <iostream>
+#include <string>
+#include <set>
+#include <vector>
+#include "ThemeData.h"
+
+int main() {
+    int passed = 0;
+
+    // Test 1: Dark theme — all semantic colors defined (no pure black/white)
+    {
+        auto t = getDefaultDarkTheme();
+        assert(!t.bg.isPureBlack()); assert(!t.bg.isPureWhite());
+        assert(!t.text.isPureBlack()); assert(!t.text.isPureWhite());
+        assert(!t.accent.isPureBlack()); assert(!t.accent.isPureWhite());
+        assert(!t.syntaxKeyword.isPureBlack());
+        assert(!t.diagError.isPureBlack());
+        std::cout << "Test 1 PASSED: Dark theme semantic colors defined\n";
+        passed++;
+    }
+
+    // Test 2: Light theme — all semantic colors defined
+    {
+        auto t = getDefaultLightTheme();
+        assert(!t.bg.isPureBlack()); assert(!t.bg.isPureWhite());
+        assert(!t.text.isPureBlack()); assert(!t.text.isPureWhite());
+        assert(!t.accent.isPureBlack()); assert(!t.accent.isPureWhite());
+        std::cout << "Test 2 PASSED: Light theme semantic colors defined\n";
+        passed++;
+    }
+
+    // Test 3: Full JSON roundtrip — dark theme
+    {
+        auto orig = getDefaultDarkTheme();
+        auto j = orig.toJson();
+        auto restored = WhetstoneTheme::fromJson(j);
+        assert(restored.name == orig.name);
+        assert(restored.bg.toHex() == orig.bg.toHex());
+        assert(restored.text.toHex() == orig.text.toHex());
+        assert(restored.accent.toHex() == orig.accent.toHex());
+        assert(restored.syntaxKeyword.toHex() == orig.syntaxKeyword.toHex());
+        assert(restored.diagError.toHex() == orig.diagError.toHex());
+        assert(restored.fontSizePrimary == orig.fontSizePrimary);
+        std::cout << "Test 3 PASSED: Full JSON roundtrip (dark)\n";
+        passed++;
+    }
+
+    // Test 4: Full JSON roundtrip — light theme
+    {
+        auto orig = getDefaultLightTheme();
+        auto j = orig.toJson();
+        auto restored = WhetstoneTheme::fromJson(j);
+        assert(restored.name == orig.name);
+        assert(restored.bg.toHex() == orig.bg.toHex());
+        assert(restored.syntaxString.toHex() == orig.syntaxString.toHex());
+        assert(restored.diagWarning.toHex() == orig.diagWarning.toHex());
+        std::cout << "Test 4 PASSED: Full JSON roundtrip (light)\n";
+        passed++;
+    }
+
+    // Test 5: toJson includes all fields
+    {
+        auto j = getDefaultDarkTheme().toJson();
+        // Check all expected keys
+        std::vector<std::string> required = {
+            "name", "bg", "bgAlt", "bgPanel", "bgPopup",
+            "text", "textDim", "textAccent", "textError", "textWarning", "textSuccess",
+            "border", "borderFocused", "accent", "accentHover", "accentActive",
+            "selectionBg", "selectionText", "scrollbar", "scrollbarHover",
+            "diagError", "diagWarning", "diagInfo", "diagHint",
+            "syntaxKeyword", "syntaxString", "syntaxNumber", "syntaxComment",
+            "syntaxFunction", "syntaxType", "syntaxOperator", "syntaxAnnotation",
+            "fontSizePrimary", "fontSizeUI", "fontSizeSmall"
+        };
+        for (const auto& key : required) {
+            assert(j.contains(key));
+        }
+        std::cout << "Test 5 PASSED: toJson includes all " << required.size() << " fields\n";
+        passed++;
+    }
+
+    // Test 6: fromJson with missing fields uses defaults
+    {
+        nlohmann::json j;
+        j["name"] = "Partial";
+        j["bg"] = "#ff0000";
+        auto t = WhetstoneTheme::fromJson(j);
+        assert(t.name == "Partial");
+        assert(t.bg.toHex() == "#ff0000");
+        // Missing fields fall back to dark theme defaults
+        auto def = getDefaultDarkTheme();
+        assert(t.text.toHex() == def.text.toHex());
+        assert(t.syntaxKeyword.toHex() == def.syntaxKeyword.toHex());
+        std::cout << "Test 6 PASSED: fromJson with partial data uses defaults\n";
+        passed++;
+    }
+
+    // Test 7: fromJson with empty object gives defaults
+    {
+        auto t = WhetstoneTheme::fromJson(nlohmann::json::object());
+        assert(t.name == "Custom"); // default name
+        auto def = getDefaultDarkTheme();
+        assert(t.bg.toHex() == def.bg.toHex());
+        assert(t.fontSizePrimary == def.fontSizePrimary);
+        std::cout << "Test 7 PASSED: Empty JSON gives default values\n";
+        passed++;
+    }
+
+    // Test 8: All 8 syntax colors distinct in dark theme
+    {
+        auto t = getDefaultDarkTheme();
+        std::vector<std::string> hexes = {
+            t.syntaxKeyword.toHex(), t.syntaxString.toHex(),
+            t.syntaxNumber.toHex(), t.syntaxComment.toHex(),
+            t.syntaxFunction.toHex(), t.syntaxType.toHex(),
+            t.syntaxOperator.toHex(), t.syntaxAnnotation.toHex()
+        };
+        // At least 6 distinct colors (keyword == function is OK by design)
+        std::set<std::string> unique(hexes.begin(), hexes.end());
+        assert(unique.size() >= 6);
+        std::cout << "Test 8 PASSED: " << unique.size() << " distinct syntax colors\n";
+        passed++;
+    }
+
+    // Test 9: Color4 arithmetic precision
+    {
+        Color4 c = Color4::fromHex("#7aa2f7");
+        // Round-trip should be stable
+        std::string hex1 = c.toHex();
+        Color4 c2 = Color4::fromHex(hex1);
+        assert(c2.toHex() == hex1);
+        // Alpha channel preserved
+        Color4 ca = Color4::fromHex("#7aa2f780");
+        assert(ca.a < 0.6f); // 0x80 = 128 → 128/255 ≈ 0.502
+        std::cout << "Test 9 PASSED: Color4 hex precision and alpha\n";
+        passed++;
+    }
+
+    // Test 10: Font size customization via JSON
+    {
+        nlohmann::json j;
+        j["name"] = "BigFont";
+        j["fontSizePrimary"] = 20.0f;
+        j["fontSizeUI"] = 18.0f;
+        j["fontSizeSmall"] = 12.0f;
+        auto t = WhetstoneTheme::fromJson(j);
+        assert(t.fontSizePrimary == 20.0f);
+        assert(t.fontSizeUI == 18.0f);
+        assert(t.fontSizeSmall == 12.0f);
+        std::cout << "Test 10 PASSED: Font size customization via JSON\n";
+        passed++;
+    }
+
+    // Test 11: Dark vs light theme contrast
+    {
+        auto dark = getDefaultDarkTheme();
+        auto light = getDefaultLightTheme();
+        // Dark: bg is dark, text is light
+        assert(dark.bg.r < 0.2f && dark.text.r > 0.7f);
+        // Light: bg is light, text is dark
+        assert(light.bg.r > 0.8f && light.text.r < 0.2f);
+        // Both have accent colors
+        assert(!dark.accent.isPureBlack());
+        assert(!light.accent.isPureBlack());
+        std::cout << "Test 11 PASSED: Dark/light contrast correct\n";
+        passed++;
+    }
+
+    // Test 12: Theme selection/scrollbar/popup colors defined
+    {
+        auto t = getDefaultDarkTheme();
+        assert(!t.selectionBg.isPureBlack());
+        assert(!t.scrollbar.isPureBlack());
+        assert(!t.bgPopup.isPureBlack());
+        assert(!t.accentHover.isPureBlack());
+        assert(!t.accentActive.isPureBlack());
+        // Hover should be lighter than accent for dark theme
+        assert(t.accentHover.r >= t.accent.r || t.accentHover.g >= t.accent.g);
+        std::cout << "Test 12 PASSED: Selection/scrollbar/popup colors defined\n";
+        passed++;
+    }
+
+    std::cout << "\nResults: " << passed << "/12\n";
+    assert(passed == 12);
+    return 0;
+}