Those are policy decisions, not technical issues. Obviously *script and CSS mods work today. But running arbitrary *script on choiceofgames.com is a security risk, so it’s very unlikely that we’ll ever allow that in games we host.
As for CSS mods, I guess we could? But that gets back to the question of what our games should look like. If we do a fancy UI redesign, would we stick with whatever custom CSS you provide? Or would we blow it away and replace it with our stuff?