geobomatic,
Can you please clarify if you are looking to delete the imported editor themes in your workspace?
If yes, then please follow these steps to delete the themes manually from the workspace :
1. Edit the com.github.eclipsecolortheme.prefs
file located in the workspace dir\.metadata\.plugins\org.eclipse.core.runtime\.settings
folder.
2. Remove the entries importedColorTheme
from the file. For example, if you have imported two themes into your workspace, then you will see two entries, importedColorTheme1 and importedColorTheme2. Restart the IDE to apply the changes.
Hope this helps. Please let us know how it works for you.
–Swapna
Genuitec Support