mediawiki.ui: Update color palette

New variables have been added to encourage reuse of the preferred
color palette.

See https://trello.com/c/IRqbu8p4/15-color-swatches

Also replacing deprecated LESS variables.

Change-Id: I03620cc4acb1ea0c7c0d2e951a9913396276c27a
Merge "mediawiki.ui: Update color palette"