Hi @Ansar ,
Do you want to change the background color here?

Firstly, let me explain why this setting is disable in sharepoint custom form.
Actually, this background color only affect the app's icon color and the app's loading screen color.


It will not affect the color inside the app.
However, sharepoint custom form is different. It is just a form, not a standalone app.
It does not has an app icon or need and loading screen.
So this feather is disabled in sharepoint custom form.
I suggest you learn more about the differences between powerapps app and sharepoint custom form.
Here's a doc about this for your reference: (please notice the Q&A part)
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/customize-list-form
Secondly, could you tell me the detailed place's background that you want to change in your custom form?
If you want to change the total screen's background, you just need to change the FormScreen1's Fill:

If you want to change the form's background, you could set SharePointForm1's Fill:

Best regards,