If you want to restore the default theme on the Samsung device, the procedure is slightly different than when adding a new theme. To reset the theme to default, open the "My Data" tab, where you can select the downloaded themes as well as the default Samsung theme.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Themes
Step 3:  Open the  menu
Step 4:  Tap on  My Stuff
Step 5:  Tap on  Default  Theme
Step 6:  Confirm with  Apply
  1. Open the Settings
  2. Tap on Themes
  3. Open the menu
  4. Tap on My Stuff
  5. Tap on Default Theme
  6. Confirm with Apply

Samsung Instructions