[BUGFIX] Enforce light mode in EXT:styleguide examples

EXT:styleguide is not fully compatible to dark mode environments. For
the time being, light mode is enforce in examples until further
groundwork is done.

Resolves: #102310
Releases: main
Change-Id: I098929883fbab6969e7bdd3d4af8c9fd487f8556
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81675


Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Garvin Hicking <gh@faktor-e.de>