Here is a list of steps to customize and format page backgrounds and templates in OneNote for Windows 10:

1. Open the page or template you want to customize
2. Select the Page Color option in the Page Color drop-down in the ribbon
3. Choose the color you want from the list of predefined colors
4. Double-click on the page that you want to format and then selectFormat Page from the drop-down menu
5. Choose the image for your page background from the Background window list
6. Set the brightness and contrast options to adjust the page accordingly
7. Click Apply to set changes