Remove the viewerCssTheme-option, since it's partially broken

The `viewerCssTheme`-implementation has always been somewhat hacky, and now it's also *partially* broken ever since we've started using CSS nesting.
Trying to support nested media queries would thus require a lot more parsing of the CSS rules, which seems inefficient and thus generally undesirable.[1]

As discussed on Matrix, let's try to remove the `viewerCssTheme`-option and see if there's any (significant) fallout from this.

---
[1] If this option is brought back, it seems to me that it (in Firefox) should probably be set through the platform-code that handles theming.

web/app.js

@@ -85,12 +85,6 @@ const ViewOnLoad = {
   INITIAL: 1,
 };
 
-const ViewerCssTheme = {
-  AUTOMATIC: 0, // Default value.
-  LIGHT: 1,
-  DARK: 2,
-};
-
 class DefaultExternalServices {
   constructor() {
     throw new Error("Cannot initialize DefaultExternalServices.");
@@ -233,7 +227,6 @@ const PDFViewerApplication = {
     if (AppOptions.get("pdfBugEnabled")) {
       await this._parseHashParams();
     }
-    this._forceCssTheme();
 
     // Ensure that the `L10n`-instance has been initialized before creating
     // e.g. the various viewer components.
@@ -347,47 +340,6 @@ const PDFViewerApplication = {
     }
   },
 
-  /**
-   * @private
-   */
-  _forceCssTheme() {
-    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 || [];
-      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 (cssTheme === ViewerCssTheme.LIGHT) {
-            styleSheet.deleteRule(i);
-            return;
-          }
-          // cssTheme === ViewerCssTheme.DARK
-          const darkRules =
-            /^@media \(prefers-color-scheme: dark\) {\n\s*([\w\s-.,:;/\\{}()]+)\n}$/.exec(
-              rule.cssText
-            );
-          if (darkRules?.[1]) {
-            styleSheet.deleteRule(i);
-            styleSheet.insertRule(darkRules[1], i);
-          }
-          return;
-        }
-      }
-    } catch (reason) {
-      console.error(`_forceCssTheme: "${reason?.message}".`);
-    }
-  },
-
   /**
    * @private
    */

web/app_options.js

@@ -218,11 +218,6 @@ const defaultOptions = {
     value: 1,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
-  viewerCssTheme: {
-    /** @type {number} */
-    value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("CHROME") ? 2 : 0,
-    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
-  },
   viewOnLoad: {
     /** @type {boolean} */
     value: 0,