Administration: Remove non-color related styling from Modern color scheme.

Color schemes should only affect colors, to avoid diverging from a consistent and predictable layout, especially if core CSS rules subsequently change.

Follow-up to [48277], [48286], [48345].

Props Joen, johnbillion, afercia, johnjamesjacoby.
Merges [48862] to the 5.5 branch.
Fixes #51127. See #50504.
Built from https://develop.svn.wordpress.org/branches/5.5@48863


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48625 1a063a9b-81f0-0310-95a4-ce76da25c4cd