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/01-foundation/_themes.css

@@ -186,7 +186,7 @@
   left: 2rem;
   width: 50px;
   height: 50px;
-  background: var(--black-bar);
+  background: var(--black-bar, #2b2b2b);
   color: white;
   border: none;
   border-radius: 50%;
@@ -348,6 +348,6 @@
 @media (prefers-color-scheme: light) {
   [data-color-theme="auto"] img[src*="livgolf"] {
     filter: none;
-    border-color: var(--icon-border) !important; /* Reset to normal border */
+    border-color: var(--icon-border, #ddd) !important; /* Reset to normal border */
   }
 }