theme: Reorganize CSS selectors to make more sense
The first commit changes the import order of widget stylesheets to make the theme development easier. Originally, as I wrote in the earlier comment, I wanted to rename the stylesheets to make them more understandable, but I didn't do that this time to avoid merge conflicts with other MRs as much as possible.
The rest of the commits place CSS selectors in more appropriate stylesheets based on the source JS and the previous _common.scss. Since I'm not the developer, I may made some mistakes and/or oversights...
Edited by nana-4