When the help content is displayed in the external browser the system
preferences about system wide dark or light appearance should be
respected.

There's a CSS standard coming for this: the prefers-color-scheme
media query (see
https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme ).

The CSS of the help content should incorporate this media
query.

Change-Id: I7090ff35d34c24cdf029fd6210a1b517ba23c465
Signed-off-by: Matthias Becker <ma.becker@sap.com>