Custom CSS is possible in MoodleCloud
: Site administration -> Appearence -> Themes -> Your current theme
If you have any custom CSS then put it here, then we can see if there might be a syntax error in it.
As custom CSS from that place would be theme-specific, you can try to switch to a different theme, and see if the graphics return. If they do, then the error is in the theme somewhere. If it doesn't, then the error is somewhere else.