Did you take a look in the theme folder on your filesystem? (e.g. {projectfolder}\theme). Additional themes downloaded from the AppStore can be found there as .zip files, the (default) theme should be there as well as a unzipped version (e.g. index.html and /ui folder).
If your theme-folder looks as I describe above, try to delete the .zip file and see in the modeler if you can go back to the (default) theme.
May be you might need to clear the cache of your browser in case you changed to another theme and you have deleted the united theme from the themes folder