fix: punctuation symbols not visible in dark mode syntax highlighting (#576)

Add explicit color styling for `.highlight .p` (punctuation tokens) in both light and dark modes using Shibuya theme's `--gray-12` variable. This ensures brackets, parentheses, commas, and other punctuation symbols are properly visible when viewing code examples in dark mode.

Author: glendonyeo

docs/_static/syntax-highlighting.css

@@ -112,6 +112,10 @@ html.light .highlight .sx {  /* String.Other */
   color: #d73a49 !important;
 }
 
+html.light .highlight .p {   /* Punctuation */
+  color: var(--gray-12) !important;
+}
+
 /* =====================================================
  * Dark Mode Syntax Highlighting
  * ===================================================== */
@@ -208,6 +212,10 @@ html.dark .highlight .sx {   /* String.Other */
   color: #a5d6ff !important;
 }
 
+html.dark .highlight .p {    /* Punctuation */
+  color: var(--gray-12) !important;
+}
+
 /* =====================================================
  * Common Styles (Both Themes)
  * ===================================================== */