Add a light mode

[BuJo]

Add a way to control the look of tuwat.

[DerGeras]

At least in terms of contrast, one could argue that the yellow of the warning line could be changed, or some Border added to the whole "What" Block. But after staring at it for a few seconds, I found what was actually bothering me, which is not present in the Darf Theme:
The White Font is very close to the light backgrund, and therefore a bit hard to read at a glance.
So I played around with it, and found that just adding a bit of padding to the left side fixed it for me: