Merge pull request #2 from MahApps/master

Merge MahApps master
Style changes for Dark Theme, Codeformatting

-) Day Header and Today Selection didn't look good with Dark Theme
-) Replaced Color Resources with Brush Resources where Possible
Tooltip: Background Color is now the right color for the theme.
Dark Theme:  Background=black, Text=white, border-color=white
Light Theme: Background=white, Text=black, border-color=black
Merge pull request #687 from xxMUROxx/hotfix/Tooltip-Style

Tooltip: Background Color is now the right color for the theme.
Merge pull request #685 from stefan-schweiger/master

Calendar style changes for Dark Theme
calendar style formatting