I think that a large part of the issue is that stylesheets have by necessity (but also unfortunately) become much more complex and larger nowadays as they have evolved to include style rules which render page content and layout differently, depending on the screen size of the browsing device.
When stylesheets first became commonplace, there was a brief trend for some sites to offer users the choice of different styles to choose from, such as light, dark, high contrast, etc, which was kinda nice, but I guess the multiple maintenance overhead soon put an end to that.
There was (is?) also the capability for browsers to optionally apply user override styles so that you could avoid the more searing color combinations of some sites, or improve accessibility with more readable or larger fonts, etc, but I'm not sure whether anyone other than the Opera of old ever took that sort of functionality seriously, sadly (perhaps there might also be some Firefox add-ons around which could do similar, if they ever survived the Great Purge of the Extensions…)?
On a related note, the excellent Dark Reader add-on is definitely worth a mention: it lets you apply either a 'dark mode' or more off-white (thankfully, death to #FFFFFF!) 'parchment mode' style to any website.