the only panel I can see is this one
JPanel content = new JPanel();
and changing the background for that worked fine for me.
if these are customized panels
private static IgralnaPlata plata = new IgralnaPlata();
private static PredogledPlata predogled = new PredogledPlata();
you're the only one with the code of the customization,
so you'd be on your own with those to find out the problem.
Note: when posting code, create another program that only has code
relevant to the problem e.g. are the labels or the menuBar or actionListener
part of the problem? If not, don't include them. Then post the stripped-down
version of your program, making sure we can copy/paste/compile/run and see
the problem as you see it.