How can I customize the proposal background image?

1) Proposals -> Proposal -> Preview



2) Click on the "Choose or edit theme" button



3) You can choose a background image from the left vertical panel. You can then download the background image from the Cover page or Inner page tab.
Do a right-click on the thumbnail image and select "Save Image As..." from the menu.



4) Download the background image, customize it, and upload it again.



5) Click on the button "Save and select theme"