We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
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;
          }
          
          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
   */
  async _initializeViewerComponents() {