[BUGFIX] htmlArea RTE: Default skin inconsistent with ExtJS xtheme-blue

The default RTE skin, used as default skin in FE, has colors
inconsistent with ExtJS skin xtheme-blue which is loaded by default
in FE.

Change-Id: I4c95c94a66ecf78fe39f163310a45d327b9007a2
Fixes: #33889
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/9007
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland