fix: improve secondary subtitle readability

config.example.jsonc

@@ -185,10 +185,10 @@
       "wordSpacing": 0, // Word spacing setting.
       "fontKerning": "normal", // Font kerning setting.
       "textRendering": "geometricPrecision", // Text rendering setting.
-      "textShadow": "0 3px 10px rgba(0,0,0,0.69)", // Text shadow setting.
-      "backgroundColor": "transparent", // Background color setting.
+      "textShadow": "0 2px 4px rgba(0,0,0,0.95), 0 0 8px rgba(0,0,0,0.8), 0 0 16px rgba(0,0,0,0.55)", // Text shadow setting.
+      "backgroundColor": "rgba(20, 22, 34, 0.78)", // Background color setting.
       "backdropFilter": "blur(6px)", // Backdrop filter setting.
-      "fontWeight": "normal", // Font weight setting.
+      "fontWeight": "600", // Font weight setting.
       "fontStyle": "normal" // Font style setting.
     } // Secondary setting.
   }, // Primary and secondary subtitle styling.

src/config/config.test.ts

@@ -64,6 +64,12 @@ test('loads defaults when config is missing', () => {
     'Inter, Noto Sans, Helvetica Neue, sans-serif',
   );
   assert.equal(config.subtitleStyle.secondary.fontColor, '#cad3f5');
+  assert.equal(config.subtitleStyle.secondary.fontWeight, '600');
+  assert.equal(
+    config.subtitleStyle.secondary.textShadow,
+    '0 2px 4px rgba(0,0,0,0.95), 0 0 8px rgba(0,0,0,0.8), 0 0 16px rgba(0,0,0,0.55)',
+  );
+  assert.equal(config.subtitleStyle.secondary.backgroundColor, 'rgba(20, 22, 34, 0.78)');
   assert.equal(config.immersionTracking.enabled, true);
   assert.equal(config.immersionTracking.dbPath, '');
   assert.equal(config.immersionTracking.batchSize, 25);

src/config/definitions/defaults-subtitle.ts

@@ -50,10 +50,10 @@ export const SUBTITLE_DEFAULT_CONFIG: Pick<ResolvedConfig, 'subtitleStyle'> = {
       wordSpacing: 0,
       fontKerning: 'normal',
       textRendering: 'geometricPrecision',
-      textShadow: '0 3px 10px rgba(0,0,0,0.69)',
-      backgroundColor: 'transparent',
+      textShadow: '0 2px 4px rgba(0,0,0,0.95), 0 0 8px rgba(0,0,0,0.8), 0 0 16px rgba(0,0,0,0.55)',
+      backgroundColor: 'rgba(20, 22, 34, 0.78)',
       backdropFilter: 'blur(6px)',
-      fontWeight: 'normal',
+      fontWeight: '600',
       fontStyle: 'normal',
     },
   },

src/renderer/style.css

@@ -673,13 +673,17 @@ body.platform-macos.layer-visible #subtitleRoot {
 }
 
 #secondarySubContainer {
+  --secondary-sub-background-color: transparent;
+  --secondary-sub-backdrop-filter: none;
   position: absolute;
   top: 40px;
   left: 50%;
   transform: translateX(-50%);
   max-width: 80%;
   padding: 10px 18px;
-  background: transparent;
+  background: var(--secondary-sub-background-color, transparent);
+  backdrop-filter: var(--secondary-sub-backdrop-filter, none);
+  -webkit-backdrop-filter: var(--secondary-sub-backdrop-filter, none);
   border-radius: 8px;
   pointer-events: auto;
 }
@@ -736,6 +740,8 @@ body.settings-modal-open #secondarySubContainer {
   transform: none;
   max-width: 100%;
   background: transparent;
+  backdrop-filter: none;
+  -webkit-backdrop-filter: none;
   padding: 40px 0 0 0;
   border-radius: 0;
   display: flex;
@@ -744,6 +750,8 @@ body.settings-modal-open #secondarySubContainer {
 
 #secondarySubContainer.secondary-sub-hover #secondarySubRoot {
   background: transparent;
+  backdrop-filter: none;
+  -webkit-backdrop-filter: none;
   border-radius: 8px;
   padding: 10px 18px;
 }
@@ -752,6 +760,12 @@ body.settings-modal-open #secondarySubContainer {
   opacity: 1;
 }
 
+#secondarySubContainer.secondary-sub-hover:hover #secondarySubRoot {
+  background: var(--secondary-sub-background-color, transparent);
+  backdrop-filter: var(--secondary-sub-backdrop-filter, none);
+  -webkit-backdrop-filter: var(--secondary-sub-backdrop-filter, none);
+}
+
 iframe.yomitan-popup,
 iframe[id^='yomitan-popup'] {
   pointer-events: auto !important;

src/renderer/subtitle-render.test.ts

@@ -347,6 +347,58 @@ test('applySubtitleStyle sets subtitle name-match color variable', () => {
   }
 });
 
+test('applySubtitleStyle stores secondary background styles in hover-aware css variables', () => {
+  const restoreDocument = installFakeDocument();
+  try {
+    const subtitleRoot = new FakeElement('div');
+    const subtitleContainer = new FakeElement('div');
+    const secondarySubRoot = new FakeElement('div');
+    const secondarySubContainer = new FakeElement('div');
+    const ctx = {
+      state: createRendererState(),
+      dom: {
+        subtitleRoot,
+        subtitleContainer,
+        secondarySubRoot,
+        secondarySubContainer,
+      },
+    } as never;
+
+    const renderer = createSubtitleRenderer(ctx);
+    renderer.applySubtitleStyle({
+      secondary: {
+        backgroundColor: 'rgba(20, 22, 34, 0.78)',
+        backdropFilter: 'blur(6px)',
+        fontWeight: '600',
+      },
+    } as never);
+
+    const secondaryStyleValues = (
+      secondarySubContainer.style as unknown as {
+        values?: Map<string, string>;
+        backgroundColor?: string;
+        backdropFilter?: string;
+      }
+    ).values;
+    assert.equal(
+      secondaryStyleValues?.get('--secondary-sub-background-color'),
+      'rgba(20, 22, 34, 0.78)',
+    );
+    assert.equal(secondaryStyleValues?.get('--secondary-sub-backdrop-filter'), 'blur(6px)');
+    assert.equal(
+      (secondarySubContainer.style as unknown as { backgroundColor?: string }).backgroundColor,
+      undefined,
+    );
+    assert.equal(
+      (secondarySubContainer.style as unknown as { backdropFilter?: string }).backdropFilter,
+      undefined,
+    );
+    assert.equal((secondarySubRoot.style as unknown as { fontWeight?: string }).fontWeight, '600');
+  } finally {
+    restoreDocument();
+  }
+});
+
 test('computeWordClass adds frequency class for single mode when rank is within topX', () => {
   const token = createToken({
     surface: '猫',
@@ -819,6 +871,42 @@ test('JLPT CSS rules use underline-only styling in renderer stylesheet', () => {
     /-webkit-text-fill-color:\s*var\(--subtitle-hover-token-color,\s*#f4dbd6\)\s*!important;/,
   );
 
+  const secondaryContainerBlock = extractClassBlock(cssText, '#secondarySubContainer');
+  assert.match(
+    secondaryContainerBlock,
+    /background:\s*var\(--secondary-sub-background-color,\s*transparent\);/,
+  );
+  assert.match(
+    secondaryContainerBlock,
+    /backdrop-filter:\s*var\(--secondary-sub-backdrop-filter,\s*none\);/,
+  );
+
+  const secondaryRootBlock = extractClassBlock(cssText, '#secondarySubRoot');
+  assert.match(secondaryRootBlock, /-webkit-text-stroke:\s*0\.45px rgba\(0,\s*0,\s*0,\s*0\.7\);/);
+  assert.match(
+    secondaryRootBlock,
+    /text-shadow:\s*0 2px 4px rgba\(0,\s*0,\s*0,\s*0\.95\),\s*0 0 8px rgba\(0,\s*0,\s*0,\s*0\.8\),\s*0 0 16px rgba\(0,\s*0,\s*0,\s*0\.55\);/,
+  );
+
+  const secondaryHoverBaseBlock = extractClassBlock(
+    cssText,
+    '#secondarySubContainer.secondary-sub-hover #secondarySubRoot',
+  );
+  assert.match(secondaryHoverBaseBlock, /background:\s*transparent;/);
+
+  const secondaryHoverVisibleBlock = extractClassBlock(
+    cssText,
+    '#secondarySubContainer.secondary-sub-hover:hover #secondarySubRoot',
+  );
+  assert.match(
+    secondaryHoverVisibleBlock,
+    /background:\s*var\(--secondary-sub-background-color,\s*transparent\);/,
+  );
+  assert.match(
+    secondaryHoverVisibleBlock,
+    /backdrop-filter:\s*var\(--secondary-sub-backdrop-filter,\s*none\);/,
+  );
+
   assert.doesNotMatch(
     cssText,
     /body\.layer-visible\s+#secondarySubContainer\s*\{[^}]*display:\s*none/i,

src/renderer/subtitle-render.ts

@@ -155,6 +155,33 @@ const CONTAINER_STYLE_KEYS = new Set<string>([
   '-webkit-backdrop-filter',
 ]);
 
+function resolveSecondaryBackgroundColor(declarations: Record<string, unknown>): string {
+  for (const key of ['backgroundColor', 'background']) {
+    const value = declarations[key];
+    if (typeof value === 'string' && value.trim().length > 0) {
+      return value.trim();
+    }
+  }
+
+  return 'transparent';
+}
+
+function resolveSecondaryBackdropFilter(declarations: Record<string, unknown>): string {
+  for (const key of [
+    'backdropFilter',
+    'WebkitBackdropFilter',
+    'webkitBackdropFilter',
+    '-webkit-backdrop-filter',
+  ]) {
+    const value = declarations[key];
+    if (typeof value === 'string' && value.trim().length > 0) {
+      return value.trim();
+    }
+  }
+
+  return 'none';
+}
+
 function getFrequencyDictionaryClass(
   token: MergedToken,
   settings: FrequencyRenderSettings,
@@ -700,9 +727,17 @@ export function createSubtitleRenderer(ctx: RendererContext) {
       secondaryStyleDeclarations,
       CONTAINER_STYLE_KEYS,
     );
-    applyInlineStyleDeclarations(
-      ctx.dom.secondarySubContainer,
-      pickInlineStyleDeclarations(secondaryStyleDeclarations, CONTAINER_STYLE_KEYS),
+    const secondaryContainerStyleDeclarations = pickInlineStyleDeclarations(
+      secondaryStyleDeclarations,
+      CONTAINER_STYLE_KEYS,
+    );
+    ctx.dom.secondarySubContainer.style.setProperty(
+      '--secondary-sub-background-color',
+      resolveSecondaryBackgroundColor(secondaryContainerStyleDeclarations),
+    );
+    ctx.dom.secondarySubContainer.style.setProperty(
+      '--secondary-sub-backdrop-filter',
+      resolveSecondaryBackdropFilter(secondaryContainerStyleDeclarations),
     );
 
     if (secondaryStyle.fontFamily) {