Add CSS variables for code/editor surfaces

Introduce new CSS variables (--bg-input, --bg-code, --code-text, --code-gutter) in base.css for both dark and light themes and replace hardcoded colors across components.css to use these variables. Updates touch textareas, filter inputs, diff/code previews, markdown code blocks and git output to centralize theming. Also add light-mode styles for modal backdrops, syntax token colors, and diff gutter border to improve light/dark parity and maintainability. Files changed: frontend/css/base.css, frontend/css/components.css.

frontend/css/base.css

@@ -24,6 +24,12 @@
   --success: #3FB950;
   --danger: #F85149;
 
+  /* Code / editor surfaces */
+  --bg-input: rgba(0, 0, 0, 0.28);
+  --bg-code: #0B0D11;
+  --code-text: #E6EDF3;
+  --code-gutter: #7D8590;
+
   /* Shape */
   --radius-md: 7px;
   --radius-lg: 11px;
@@ -48,6 +54,12 @@
   --success: #16a34a;
   --danger: #dc2626;
   --shadow: 0 8px 28px rgba(197, 123, 39, 0.12);
+
+  /* Code / editor surfaces */
+  --bg-input: rgba(255, 255, 255, 0.85);
+  --bg-code: #F8F3EC;
+  --code-text: #2B1D0E;
+  --code-gutter: #8B7A60;
 }
 
 * {
@@ -142,7 +154,7 @@ textarea {
   padding: 5px 10px;
   border-radius: var(--radius-md);
   border: 1px solid var(--border);
-  background: rgba(0, 0, 0, 0.28);
+  background: var(--bg-input);
   color: var(--text-main);
   font-size: 13px;
   transition: border-color 0.12s ease, box-shadow 0.12s ease;