By using colors from KColorScheme instead of hardcoding them we make the default
KMail colors work with both dark and light system color themes.
This fixes color of unread/important/todo messages in message list, quotation
colors in message view and PGP signature status boxes. It removes the possibility
to configure custom colors for PGP signature status boxes, but on the other hand
the color boxes and the text now have correct contrast on all color themes.
REVIEW: 125128