TabControl's TabItems aren't properly updated on attempt to change the theme palette variation runtime until any of the items is selected. Check the following video - https://www.screencast.com/t/jMj0EZqFfy.

There are several possible workarounds which include:

• Clearing all merged resource dictionaries and re-merging them
• Increasing and decreasing the height of the MainWindow by 1 after loading the chosen palette to force the refresh