@tymo614 RetroPie themes are stored on Github repos so you can download a zip, extract it and copy it to your USB stick. If you wish to restore the default Carbon theme, then you can download it at: https://github.com/RetroPie/es-theme-carbon
Once you have the theme on your USB stick, then place the es-theme-carbon-master folder in the /home/pi/.emulationstation/themesfolder on your pi. When you restart EmulationStation, the theme should be available under UI settings in the ES menu.
You can find many of the themes at the RetroPie Github page: https://github.com/RetroPie, themes repos begin es-theme-{name of theme}.
If you can't find a theme, then you can check the source here and google for it.