When using Dark Reader to change the CSS for contrast and readability, it breaks the CSS lessons in Responsive Web Design that check for color. This is far from obvious as the color is still a shade of red to the user.
I recommend either 1. allowing different shades for those lessons, 2. checking the code used instead of the style in the results or 3. do the tests on the server side. This issue has not only caused problems for users like me who use extensions but also to users on older browsers or on mobile.
The least that can be done is to warn users that tests are done locally, and that changing the environment (browser, extensions, platform) can help fix issues unrelated to the code produced.