Why did my form suddenly change to a "theme"?

My form "story upload" was working fine, then all of a sudden it had a new font, a background image of food, and other changes. I didn't select a "theme" and trying to set the theme to "default" gives me this food thing. Help?

I would like to suggest changing your theme to a blank theme: Reset theme styles

Then save the form and confirm that the changes have been removed.

Then apply your own theme once again over that reset theme, or just apply your own styles over that one - it does not have any styles in it, so everything that you do is the only style that would be added to it.

Do let us know how it goes.