Opacity of resource boxes

[Marco Martin]

On Wednesday 28 March 2012, Thomas Pfeiffer wrote:
> and color (maybe with a little tweaking) would work for resource boxes as
> well? I don't know. But there is still a lot of room between the current
> plain almost-completely-opaque grey and the almost complete transparency
> in your screenshot.
> 
> I'm confident that we can find values which make text readable without
> looking like a piece of grey.

I have committed an attempt about it to master, I am quite unsure about the 
look tough

so to everybody: please test, paying attention to the first reaction ;)

-- 
Marco Martin