Many users I think have their CSS mods all in one giant CSS file, which would make it prone to breaking other mods when installed. Mod files should be for specific and clear uses so they can easily be enabled without breaking other mods.
As a user, you would no longer have to deal with that at all, actually, since the store would make it easy to toggle mods with specific features.
Also, if you still want to copy/paste some CSS because someone posted it in the forum without intending to distribute it, then I can for sure add an easy UI to do that as well.
The only thing you would be changing is the option to enable modding and the path.
No, sometimes mods require specific settings to be enabled, for exemple here's mine : https://forum.vivaldi.net/topic/105134/φ-phi-the-ultimate-vertical-experience-theme-for-vivaldi-made-with-attention-to-details
users can do that themselves if necessary
That's actually something I'd like to avoid as well, as most people don't know how or don't dare to navigate to vivaldi:experiements, same for chrome://flags on Chromium, about:config on Firefox, etc.
The objective here is to make CSS mod management as easy and popular as browser extensions.