84a66d82d36d554c1844acff11d1a62f10c174ed added support for light and
dark themes, but dark themes weren't enabled, as diff styles have
to be adjusted to correspond with dark colors.
101a8c4b65ebee4d54afddad10db05f5b9698ba9 reverted the preparation
for dark themes support.
This change enables support for subset of dark themes and switch
dynamicaly the diff styles to dark colors, when dark theme was
selected.
Change-Id: Ic220baf8f19366c04520b61e893a0d11912805ef
51b373679e