Open the Projects Setting panel - Ctrl+Shift+P.
In the 2D Background click on the colour box after Background Color and select one from the selection window that opens.

You can read about the various options in the Project Settings
here in the iClone 7 Manual.
:)
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.