fix: make code blocks respond to dark mode theme toggle (#200)

Replace hardcoded colors in .content pre with CSS variables
(--color-code-bg, --color-code-fg) that adapt to the selected theme.

Closes #159

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

astro-site/src/styles/global.css

@@ -19,6 +19,8 @@
   --color-border: #0000001f;
   --color-link: #00796b;
   --color-focus: #00796b;
+  --color-code-bg: #f5f5f5;
+  --color-code-fg: #212121;
 
   /* Spacing */
   --space-xs: 0.25rem;
@@ -55,6 +57,8 @@
   --color-border: #ffffff1f;
   --color-link: #4dd0e1;
   --color-focus: #4dd0e1;
+  --color-code-bg: #2a2a2a;
+  --color-code-fg: #e0e0e0;
 }
 
 /* Reset */
@@ -531,8 +535,8 @@ a:hover {
 }
 
 .content pre {
-  background: #1e1e1e;
-  color: #e0e0e0;
+  background: var(--color-code-bg);
+  color: var(--color-code-fg);
   border-radius: 4px;
   padding: var(--space-md);
   margin-bottom: var(--space-lg);