Fixing a few issues in ThemeManager

[batzen]

• ThemeManager did not look at merged dictionaries the way wpf did. wpf looks at them in a reverse order and so should we.
• StringComparison should be OrdinalIgnoreCase
• Switched to nameof instead of hard coded values
• Switched to string interpolation instead of string.format
• Switched to null conditional operator instead of SafeRaise
• Added some missing documentation
• Removed "metro" from documentation to ease sharing of code with Fluent.Ribbon

[punker76]

@batzen 👍