Ahhh, apologies for misreading your suggested usage of yellow/black.
But I still don't like it.
I'm not aware of other parts of Calibre making use of coloured backgrounds so I don't want to start a disturbing trend here, I find too much colour to be garish. Kovid suggested blue and as it will become part of Calibre one day his vote counts the most for not spending the next 3 days going back and forth with various other permutations. Life is too short and my todo list is too long!
If you don't like it you can always edit the code, or send me a petition of no less than a thousand signatures...
