Format less files

[Hirse]

While prettier does some formatting of less files (double to single quotes, lowercase hex colors), it doesn't do others.

Formatting in this PR that is not done by prettier:

• Empty line before rule
• Remove units for zero values (0px → 0)
• Use long form for color declarations (#ff5 → #ffff55) (This makes it easier to later regex all colors.)
• Remove empty rules