Something to take a look at is the 'Permissions' that are in place for any new themes you have added. If the new themes are in /usr/share/themes, then 'Permission' should look like the picture below. If they do not, you need to alter the 'Permissions' to match the picture.
If, however, they have been placed in your home directory (to check, you must go to your 'home' directory and select 'View Hidden Files' from the 'View' option in your Nemo menu), then 'Permissions' should be set to ensure there is no 'root' access involved.
If you have to make any changes to the 'Permissions' area, in either directory, you must, of course, enter the necessary folder with 'root privileges' by using Nemo to navigate to where you need to go, then right click on the folder you want and select 'Open as Root'. This gives you the access to change the 'Permissions' to the appropriate settings.
If this is not the problem, someone more skilled than me will have to chime in with an answer...