How is it possible to patch this while retaining the ability for users to make custom themes? I'm genuinely curious what the right solution is. CSS is practical

How is it possible to patch this while retaining the ability for users to make custom themes? I'm genuinely curious what the right solution is. CSS is practically a programming language at this point (isn't it Turing complete?) so it seems hard (if not impossible) to accurately identify all of the features that might constitute a vulnerability when the script is deliberately loaded and run by a user. Is there a way to tell a browser "this stylesheet can't make external requests"? Are they manually parsing the stylesheet to look for the naughty selector (on the textbox element, or on $=, etc.)? Update: Oh, it looks like this is being entered via an input field in the settings meant to specify a color code. So not actually a fully-fledged user stylesheet. On that note, does Slack support those?