How do I insert background image in XLSForm.
Secondly how can I add background colour to my app using XLSForm
You can add images to your questions. There is not a setting to configure the background colour.
See the docs on Form Styling for more details.
Welcome to the ODK forum, @nchibuzo74! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!