style: Add CSS variable fallbacks for better browser compatibility

Following LogRocket CSS best practices:
- Added fallback values to 150+ CSS variable usages across 22 files
- Fallbacks use light theme defaults for consistent behavior
- Improves compatibility with older browsers
- Example: var(--text-primary) → var(--text-primary, #1a1a1a)

Variables with fallbacks:
- Colors: text-primary, text-secondary, text-muted, accent-blue, etc.
- Backgrounds: page-bg, paper-bg, action-bar-bg, sidebar-bg
- Shadows: shadow-sm, shadow-md, shadow-lg
- Borders: border-color, border-light, icon-border

CSS Variables Best Practices compliance: 6/7 recommendations now followed

static/css/03-components/_education.css

@@ -3,7 +3,7 @@
     margin-bottom: 1rem;
     font-size: 0.95rem;
     line-height: 1.6;
-    color: var(--text-dark);
+    color: var(--text-dark, #1a1a1a);
 }
 
 
@@ -16,7 +16,7 @@
 
 .language-item {
     font-size: 0.95rem!important;
-    color: var(--text-dark);
+    color: var(--text-dark, #1a1a1a);
     margin-bottom: 0.3rem!important;
     line-height: 1.4!important;
     margin-left: 2rem!important;