Fix dark highlighting themes missing base text color

Dark syntax highlighting themes like zenburn and espresso define
text-color at the top level but omit "Normal" from text-styles,
so generateThemeCssClasses() never emits a span-level color rule.
When layered with a light theme that does (e.g. kate emits
code.sourceCode > span { color: #1f1c1b; }), the light theme's
near-black text persists in dark mode.

Add a span-level $code-block-color rule in _bootstrap-rules.scss
alongside the existing container-level rules, following the same
pattern used for background-color. Since light and dark are
separate compiled stylesheets toggled by JS, the active theme's
rule applies without specificity conflicts.

Fixes #14099

Author: jdonaldson

src/resources/formats/html/bootstrap/_bootstrap-rules.scss

@@ -863,6 +863,14 @@ div.sourceCode {
   div.sourceCode pre.sourceCode {
     color: $code-block-color;
   }
+  // Themes that omit "Normal" from text-styles (e.g. zenburn, espresso)
+  // don't generate a span-level color rule in the syntax highlighting CSS.
+  // When layered with a light theme that does, the light theme's near-black
+  // span color persists in dark mode. This ensures a base span color is set.
+  pre > code.sourceCode > span,
+  code.sourceCode > span {
+    color: $code-block-color;
+  }
 }
 
 pre.sourceCode {