Mismatched parens background color now dark on dark backgrounds.

The color is not configurable, but on dark backgrounds, the color is now
dark magenta, rather than magenta.

Task-number: QTCREATORBUG-9025

Change-Id: I9925e598b30a13c8b9691d6e0e576c0087df38da
Reviewed-by: Orgad Shaneh <orgads@gmail.com>