Unless I have missed something, there appears to be no way within eM Cient to remove imported Themes.
To remove imported Themes you have to go to the \AppData\Roaming\eM Client\Themes folder and manually delete the unwanted Themes.
No, you have not missed anything. There is no option to remove the imported theme from within eM Client. Deleting the file as you have described is the only way.
Edit: @cyberzork proves me wrong below. 
Thanks for that. Sometimes it is the small and obvious things like the delete key that we miss.
@Louie To remove an imported Theme from within EM Client for Windows, go to Menu / Settings / Themes and scroll down to the very bottom. Then click / highlight on the Theme that you want to remove and press the “Del” key on your keyboard. Then click “Yes” as per screenshots below.
1 Like