Allow XAML theming for Manage Themes pages

Allow the following pages to be customized via the BigBox XAML theming system:

• Manage Themes

• Manage Startup Themes

• Manage Pause Themes

Currently these views appear to be hardcoded and not overridable through Themes\[ThemeName]\Views\.

As a theme creator, having control over these pages would improve visual consistency across BigBox.