Apply Wikimedia Design Style Guide colors, other minor fixes
- Colors in https://phabricator.wikimedia.org/M82 are applied. For this, LESS CSS variables are introduced.
- The Wikidata logo is minified and their SVG margins are removed.
- border-radius is set to 2px, the value that is currently used for most MediaWiki buttons.
- Some lacking margins or padding are introduced.
- Some other minor fixes are applied.
These changes are intended to fix T192235, but finer improvements and
a later review by the design team will be needed.