Have tampermonkey installed (google), then go to greasyfork (website) I have the link here https://greasyfork.org/en and search up character ai, have fun :)). Source: about 1 year ago

If the above mentioned URL rewriter doesn't work for you (I found it hard to use myself, and never could get the rules figured out), then you could try using https://github.com/janekptacijarabaci/greasemonkey and finding a redirect script here: https://greasyfork.org/en. Source: about 1 year ago

I was thinking more greasemonkey / userscripts. Source: about 1 year ago

Https://greasyfork.org/en is sort of what you're looking for. - Source: Hacker News / over 1 year ago

Then you should rather look for simple userscripts on for example https://greasyfork.org/en then use them or convert to uBO scriptlet syntax (which should be easy). Source: over 1 year ago

Check this site, has a lot of themes to choose from (and install using plugin) https://rainglow.io/. Source: almost 3 years ago

Looks like they have themes for other editors too. Source: about 3 years ago