I'm all for it! I miss those days before FB came along with their clean blue sterile and boring design.
You can actually put dots in class names: to match class="whostyle-jacobhall.net", you have two options: .whostyle-jacobhall\2enet, and [class ~= "whostyle-jacobhall.net"].
But I think it’d be better as a separate attribute e.g. whostyle="jacobhall.net" or perhaps who-style="jacobhall.net" if you want it to be valid HTML (custom attributes are supposed to have a hyphen in them), to be matched in stylesheets as [whostyle="jacobhall.net"]. (And then possibly use something other than hostnames, as I imagine sites don’t need to exist at the root of a domain name.) This also allows a global specification of [whostyle] { all: revert } if you desire it, which can’t be done with a class attribute (not properly, anyway—unless you’re willing to countenance :is([class^="whostyle-"], [class*=" whostyle-"], [class*="\09whostyle-"], [class*="\0awhostyle-"], [class*="\0dwhostyle-"], and possibly other whitespace)).
Another issue I'm focused on right now, is how to exchange these whostyles in a way that allows for automatic moderation. @kickscondor suggested JSON as a way to format CSS in a more parse-able manner, and I think that approach might be the easiest way to trust my server to only save received CSS rules that are (relatively) safe.
I hope to cross paths again soon, -j