Fix dark highlighting themes missing base text color

Dark syntax highlighting themes like zenburn and espresso define
text-color at the top level of the theme JSON but omit "Normal"
from text-styles. generateThemeCssClasses() only checked
text-styles for the base span color, so no `code.sourceCode > span`
rule was emitted for these themes. When layered over a light theme
that does emit a span-level color (e.g., kate), the light theme's
near-black text persists in dark mode, making unhighlighted tokens
invisible.

Add a fallback after the text-styles loop: if no base text color
was generated (no "" key in tokenCssByAbbr), check the top-level
text-color and emit the same CSS rules that Normal would have
produced. Uses generateCssKeyValues() to stay consistent with the
existing code path.

Fixes #14099

Author: jdonaldson

src/command/render/pandoc-html.ts

@@ -565,6 +565,30 @@ function generateThemeCssClasses(
       }
     });
 
+    // If no "Normal" style was found in text-styles, fall back to the
+    // theme's top-level "text-color" for the base .sourceCode > span rule.
+    // Several themes (zenburn, espresso, etc.) define text-color at the
+    // top level but omit "Normal" from text-styles, which leaves the base
+    // code color unset. When dark CSS is layered on light, the light
+    // theme's near-black base color persists and makes code unreadable.
+    if (!tokenCssByAbbr[""] && themeJson["text-color"]) {
+      const cssValues = generateCssKeyValues({
+        "text-color": themeJson["text-color"],
+      });
+      if (cssValues.length > 0) {
+        toCSS("", "Normal", cssValues);
+        [
+          "pre > code.sourceCode > span",
+          "code.sourceCode > span",
+          "div.sourceCode,\ndiv.sourceCode pre.sourceCode",
+        ].forEach((selector) => {
+          otherLines.push(`\n${selector} {`);
+          otherLines.push(...cssValues);
+          otherLines.push("}\n");
+        });
+      }
+    }
+
     // Sort tokenCssLines by abbr and flatten them
     // Ensure empty abbr ("") comes first by using a custom sort function
     const sortedTokenCssLines: string[] = [];