help messages: remove green background

I suspect not everyone will agree with this change: I don't think that help messages need to be green. It's rather distracting.

Before:

After:

and if we really want people to read it, then perhaps we should increase the font-size to be the same as the text elsewhere (1em):

It's still not looking great, in my opinion (especially the background colour, it's confusing here), but we can continue to iterate.

@wintermoor @jofranz Since you use this extension, what do you think? Any strong preferences?