Recreated part of an older PR but for detecting the theme instead.
Updated thememanager again.
Removed static properties and detection of a theme from app resources.
Implemented fix from #660
BorderlessWindowBehavior throws NRE if MetroWindow has no PART_TitleBar or PART_WindowCommands
avoid double type check
Merge pull request #700 from thoemmi/ShouldNotThrowOnMissingParts

BorderlessWindowBehavior should not throw on missing parts
Merge pull request #698 from Amrykid/ProgressIndicator-bug-Sublight

Implemented fix from #660
Merge pull request #578 from Amrykid/theme-manager-detect-theme

Follow up for #522