TH_BACK was being used when drawing the 3D view even though there was no way to set the color in the preferences. The color was zero'd when moving to the new 2.8x theme. Having both gradient and background colors was confusing, especially having to use 'TH_HIGH_GRAD' for the 3D view, 'TH_BACK' for other views. Move the background color back to 'TH_BACK', 'TH_BACK_GRAD' is used when gradients are enabled. RNA is unchanged so presets don't need updating.