Aligning to WikimediaUI style guide by replacing colors with
standard color palette equivalents, merging close colors
(ex: `#E6E6E6` & `#eee` become `#eaecf0` ) and slightly increase
contrast on a few to meet WCAG 2.0 level AA criteria.
Also:
- following other code bases like MobileFrontend or OOjs UI in
using `border-radius` property instead of obsolete mixin
- removing non-existing browser vendor prefixes in mixins
- removing non-used Less variables
- bringing CSS/Less code closer to coding standards and
- simplifying Less code where applicable
Change-Id: Icb936e14ff613471ea1da6df6341ec1b0543cfaf
