Improve CSS formatting consistency

Hello. I noticed that some CSS files used spaces while tabs seem to be the norm in this repository, so I made the formatting more consistent.

However, when I used `git diff` I noticed that the `css/main.css` file was showing `^M` at the end of some lines I modified. I think this is because I am using Linux while the repository was originally developed on Windows, but it looks OK on GitHub so I'm not sure if that's an issue here. Just letting you know so you can decide if it is OK to merge.