For mozcentral use Firefox color theme instead of system theme.

See: https://bugzilla.mozilla.org/show_bug.cgi?id=1701691

external/builder/builder.js

@@ -269,6 +269,12 @@ function preprocessCSS(mode, source, destination) {
   content = expandImports(content, source);
   if (mode === "mozcentral") {
     content = removePrefixed(content, hasPrefixedMozcentral);
+    // In the mozcentral version the color theme should be based on the Firefox
+    // theme instead of the system theme.
+    content = content.replace(
+      "prefers-color-scheme",
+      "-moz-toolbar-prefers-color-scheme"
+    );
   }
   fs.writeFileSync(destination, content);
 }

web/app.js

@@ -424,20 +424,23 @@ const PDFViewerApplication = {
     try {
       const styleSheet = document.styleSheets[0];
       const cssRules = styleSheet?.cssRules || [];
+      const mediaMatcher =
+        typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")
+          ? "-moz-toolbar-prefers-color-scheme"
+          : "prefers-color-scheme";
+      const mediaRule = `(${mediaMatcher}: dark)`;
+      const mediaRegex = new RegExp(
+        `^@media \\(${mediaMatcher}: dark\\) {\\n\\s*([\\w\\s-.,:;/\\\\{}()]+)\\n}$`
+      );
       for (let i = 0, ii = cssRules.length; i < ii; i++) {
         const rule = cssRules[i];
-        if (
-          rule instanceof CSSMediaRule &&
-          rule.media?.[0] === "(prefers-color-scheme: dark)"
-        ) {
+        if (rule instanceof CSSMediaRule && rule.media?.[0] === mediaRule) {
           if (cssTheme === ViewerCssTheme.LIGHT) {
             styleSheet.deleteRule(i);
             return;
           }
           // cssTheme === ViewerCssTheme.DARK
-          const darkRules = /^@media \(prefers-color-scheme: dark\) {\n\s*([\w\s-.,:;/\\{}()]+)\n}$/.exec(
-            rule.cssText
-          );
+          const darkRules = mediaRegex.exec(rule.cssText);
           if (darkRules?.[1]) {
             styleSheet.deleteRule(i);
             styleSheet.insertRule(darkRules[1], i);