This is another change to cleanup structure, already mentioned in
https://github.com/MagicMirrorOrg/MagicMirror/pull/4019#issuecomment-3792953018
After separating default and 3rd-party modules this PR moves the
`custom.css` from the mm-owned directory `css` into user owned directory
`config`.
It has a built-in function which moves the `css/custom.css` to the new
location `config/custom.css` (if the target not exists).
Let me know if there's a majority in favor of this change.
