fix(docs): readable admonition text in dark mode (#516)

AsciiDoc admonition bodies hardcode color: rgba(0,0,0,.6) from the
asciidoctor stylesheet. The dark-mode override only darkened the block
background, leaving black text on a dark (#1f2937) panel — effectively
unreadable (contrast ~1.1:1), e.g. the "Working on an existing codebase?"
note on the Spec-Driven Development page. Override the admonition content
text to var(--color-text) and darken its left divider for dark mode.
Light mode is unchanged (rule is .dark-scoped).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: raifdmueller

website/src/styles/main.css

@@ -402,6 +402,15 @@ html {
   background: #1f2937;
 }
 
+/* The asciidoctor stylesheet hardcodes the admonition body text to
+   rgba(0,0,0,.6); on the dark admonition background that renders black-on-dark
+   and is unreadable. Override the content text and its light divider for dark
+   mode. (#516) */
+.dark .asciidoc-content .admonitionblock > table td.content {
+  color: var(--color-text);
+  border-left-color: #374151;
+}
+
 .dark .asciidoc-content hr {
   border-color: #374151;
 }