Custom css

Is it better that custom css applies to /{user}, but not to whole website?

Because if user A has { background-color: black; }, and user B has { background-color: blue; }, and if user A visits user B, it will display black background. I don't like it, I think we should add custom css for only /{user}, similar to Tumblr.

[mbmjertan]

I'm not really sure what would the best approach here be. I'll post a bit more on my ideas about custom CSS tomorrow.