During OMV3 I used a nice background image, but after the upgrade to OMV4 it did not work anymore and I did not bother ..
Today I took the time to see why it did not work anymore.. Ahhh omv-custom.css has been changed to theme-custom.css
From my old nas I found the old file, copied the only interesting part to the new file and it was working. Even my old background image was still present ..
Just as I am about to turn that old NAS off ..
Background image now under OMV5 (logo of OMV in the lower right is preserved, this is example image (cut 'n paste))
Happy to have my Whale (30TB) almost ready