Colours for 1d and 2d backgrounds can be set by applying one of the available themes.
For 1d spectra we have, "Rose", "Sky", "Cucumber" and "Sandy".
For 2d spectra we have, "Daylight", "Sunset" and "Midnight".
The colours are applied at the next screen refresh. So if acquisition or analysis is in progress, the new theme will be applied when the windows are re-drawn. Otherwise (e.g. while analysing spectra), click Display - Refresh All or Resfresh from the right-click context menu of any spectrum window.