Fix illegible code blocks: use light background to match Prism theme

The existing prism.css sets color:black on code elements. Dark
bg-stone-900 backgrounds made text invisible. Switched to bg-stone-50
with a border to work with the light Prism theme.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sxc/docs.sx

@@ -16,7 +16,7 @@
     children))
 
 (defcomp ~doc-code (&key language code)
-  (div :class "bg-stone-900 rounded-lg p-4 overflow-x-auto"
+  (div :class "bg-stone-50 border border-stone-200 rounded-lg p-4 overflow-x-auto"
     (pre :class "text-sm"
       (code :class (str "language-" (or language "lisp")) code))))