I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

const cssTheme = AppOptions.get("viewerCssTheme");
    if (
      cssTheme === ViewerCssTheme.AUTOMATIC ||
      !Object.values(ViewerCssTheme).includes(cssTheme)
    ) {
      return;
    }
    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] === mediaRule) {
          if (cssTheme === ViewerCssTheme.LIGHT) {
            styleSheet.deleteRule(i);
            return;
          }
          
          const darkRules = mediaRegex.exec(rule.cssText);