[BUGFIX] Make text of extjs buttons and IE<10 pagetree labels visible

The task #52497 introduced a regression. The font-size set in body is
overridden in ext-all-notheme.css with font-size: 0px and thus needs to
be reset again in xtheme-t3skin.css.

Resolves: #55263
Related: #52497
Releases: 6.2
Change-Id: Id17d2ac9d6482c8cc54f4f5a92b3b3ef01f39d2b
Reviewed-on: https://review.typo3.org/27166
Reviewed-by: Wouter Wolters
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter