Web Extensions can't inject CSS files on Mozilla websites for security reasons. You would have to use code in userContent.css to style all websites including these excluded websites. Most extensions have a note on their Add-ons page to inform you about this.

Ok this makes sense. Now, is there a way to always have dark mode on support sites? It literally hurts my eyes as I open it up during the night.

I have to use bookmarklets on this and another Mozilla site to get rid of white or very pale backgrounds. One such bookmarklet is "zap white backgrounds" on this page: https://www.squarefree.com/bookmarklets/zap.html If you don't like a tan background, you can change the colour name in two places towards the end of the code.

Yeah... This way the support page still flashes before I can apply the bookmarklet and I have to do it EVERY SINGLE TIME I refresh the page. I guess I'll never visit this site then.

Open the about:config page and edit the pref 'extensions.webextensions.restrictedDomains' by removing 'support.mozilla.org'. Next, click the Dark Reader toolbar button and select 'Enable on restricted pages' (Settings > Site list). That setting is only available if the new design is being previewed, which can be done by clicking 'Dev tools'. Note, it's generally not recommended to do this for security reasons.

Open the about:config page and edit the pref 'extensions.webextensions.restrictedDomains' by removing 'support.mozilla.org'. ... Note, it's generally not recommended to do this for security reasons.

To expand on this: this site has privileged access to certain information from your Firefox that normal websites cannot access. Removing it from the restrictedDomains list will allow extensions to also access that information. I am not familiar with the specific details of what that includes.

So possible data exposure is only way?

The other option is to use custom style rules in a userContent.css file, but that's a lot of effort for one site if you don't plan on becoming a frequent visitor. To get an idea of what is involved in using custom style rules files, take a look at the ShadowFox project: https://overdodactyl.github.io/ShadowFox/