Use a slightly brighter color for the line numbers in dark mode. The
new color has the same hue but a brightness of 60% as opposed to 51%
before. This suffices to make author information and line numbers far
better readable when "Color by Author" is used.

Change-Id: Id5a8c47ed38704c1cc03e3f2eed6743cc8034b6f
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>