Select a layout and choose layout colour by clicking Background pencil in right toolbar;
Select layout colour or image/video for the background;
Select "Visibility" in the right toolbar and set it to show that layout on type of screens (e.g. a widescreen, desktop, and tablet) ;
Reduce your builder screen to different version (e.g mobile) and insert another layout in the same position where the previous was;
Select that layout and choose layout colour by clicking "Background" in right toolbar;
Select different layout colour or image/video for the background;
Select "Visibility" in right toolbar and set it to show for that layout only (e.g. phone).