Fix inconsistent use of spaces and tabs

[YarmoM]

Summary
This PR fixes the inconsistent use of spaces and tabs in the main CSS file.

Description
It appears some lines use 2 spaces, some use a tab and others still use both 2 spaces followed by a tab. While this does not impact the minified version, the non-minified version will IMHO improve by using a consistent indentation.

Given that the majority of lines use an indentation of 2 spaces, I chose this as the convention. I'll leave it at @kevquirk 's discretion to overrule the chosen convention and, upon doing so, I'll update the PR.

Additional measures
To prevent future PRs from messing up the indentation, one could add automation rules with packages like prettier to either check indentation at committing and refuse if the changes do not adhere to the convention, or apply the convention at committing or later.

Most likely, these proposed measures are overkill, and could be substituted by carefully inspection of PRs.

[kevquirk]

Thanks for this Yarmo. Someone raised this a while ago, but I haven't had time to look into it, so this really helps. It seems that my text editor (Atom) hides these discrepancies for some reason, so I had no idea they were there.