In s60v3 , when you install a lot o f themes, a little part is visible on application manager but the other are installed but don't appear on application manager!
To delete this themes :
1- Firt you must delete the theme that appears in the application manager
2- you install again the theme that not appears on application manager, when it said overwrite the theme you said yes.
3- Go to application manager and you can see the theme you have re-installed and now you can delete it.

You must do this with each theme that doesnt appear in application manager but first delete all theme to have more memory for reinstalling and this solution is better than to delete on the private folder/10207114/import

good luck