Replace deprecated mediawiki.UI Less variables with current ones
Replace deprecated mediawiki.UI Less variables with current ones

Replacing variables, which have been deprecated since MW 1.24,
with fix color values for now. This is an intermediate step in
order to proceed with planned WMUI Base variables usage.
Also
 - aligning code to CSS Coding Guidelines,
 - making use of `.box-sizing()` global Less mixin and
 - removing a `:not()` selector that had a typo and was not effective.

Bug: T154362
Change-Id: Idd90a4fa56d55f545f978dc68dd86adc0e77eb53

wikimedia/mediawiki-extensions-Kartographer,wikimedia/mediawiki-extensions-Kartographer,wikimedia/mediawiki-extensions-Kartographer