Some people really, really like their fonts so tiny and colour schemes so pale or neon that i can't make out anything, or get eye strain while trying to decipher their profiles/pet profiles. I know we don't have the ability to block this in user preferences, but is this something that can be done with custom css?

nope, custom css specifically doesn't affect user and pet profiles. you might be able to do something with a browser extension, though.

you should turn this into a suggestion, many people (me included) would support it

I use firefox and through the browser you can view the page without any styling at all (view, page style, no style). Its ugly as hell, but it will make all text readable.

For other browsers, or even other versions of firefox, you'll have to google to discover how/if you can do this.