You have set a background color for your app's UI Background Color, but the changes appear not to be saved because you see a different color in the Preview.
This issue occurs because there is a Header Background image set which is a solid color. The Header Background overrides the UI Bar Background Color.
- Navigate to Design > Global Styling Tab
- Scroll down to the Header Background and click the edit icon
- Select the transparent image and click the Select button on the right
- Click the Save button to save your changes
After removing the Header Background image, the UI Bar Background Color is displayed.