You can remove the background and shadow of questions on your form within the Theme & Appearance editor.
To do this:
- Click the "Theme & Appearance" button located on the left Question Panel.

- Toggle off "Questions have background box shadow" in the quick access settings.

Alternatively, this toggle can also be accessed under UI Elements → Questions.
Advanced Styling
If you're looking for a more bespoke approach to modifying question backgrounds, this requires the use of Custom CSS (a feature exclusively available with specific pricing plans.).