Have you seen the Stylus extension? I have several default styles configured in Stylus. This improves things on the desktop quite a bit. I wish it existed on mobile.

Actually, you can use it on Android, but it does require using Firefox Nightly[1].

That does come with downsides such as Nightly being less stable and installing extensions requiring some extra setup, so it isn't the best choice for everyone. But I myself am using Stylus (which works perfectly fine) and some other not officially supported WebExtensions on mobile this way and can recommend it if you don't mind the downsides.

[1]: https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...

I used to use Stylus several years ago. Didn't it sell out to another company and start tracking and selling user's data? Or am I thinking of another similar add-on?

That was Stylish.

https://en.wikipedia.org/wiki/Stylish

Right! Thank you for the clarification.

Another useful extension is Dark Background And Light Text https://addons.mozilla.org/en-US/firefox/addon/dark-backgrou... . It's a one-button toggle to turn most text and backgrounds into colors you prefer. Great when you run across one of these accursed low-contrast sites.

Yes! Been using DB&LT for years on Firefox. It also mostly put an end to the glaring bright pages. You control the Default colors for all sites (set in Global Prefs) with five options; all are retained (the options used for already configured pages are listed in GP).

How does it compare to Dark Reader?

Dunno, I've never used that one. I may give it a shot, thanks!

You can also select your own fonts and prevent pages from overriding them. Preferences → Fonts → Advanced → untick “Allow pages to choose their own fonts, instead of your selections above”. I did this a few months ago, and I’ve seen no breakage outside icon fonts, and apparently icon fonts are deader than I thought they were—DuckDuckGo has a few unimportant ones, and beyond that the only ones I’ve seen are Google’s Material Icons font, which uses a ligation technique that’s supposed to make things better but actually makes them vastly worse.

Over all, I’m much pleased by the results of forcing Equity/Concourse/Triplicate.

> For what it's worth, Firefox lets you edit them

Would it work with a... opionated website with a hundreds of !important all over the CSS?

Is it possible to use userContent.css on mobile?

Given the partitioning of device storage, my understanding is that it's either not possible. I'm not sure the browser itself even supports userContent.css.