Now checks for missing Colour/Brushes
Now checks for missing Colour/Brushes

There is a scenario where some of the Colours/Brushes aren't in the
Resource Dictionary the app loads, this is causing the loading of a
theme to fail. Have put some checks in so that if it doesn't exist, it
should be added.

jeffwilcox/wp-thememanager,jeffwilcox/wp-thememanager