[BMR777]

Ok, where you got stuck is ok and you are on the right track. After you get to your last screenshot, click on the "adopts_settings" link on the left hand side of the page.

Then another screen will appear. You should see one option named "themeurl" in the list, along with a pencil icon beside it. Click on the pencil to edit. Then find where your custom theme url is and replace it with the path to your old theme, then submit the page and that should solve things. :)