Whilst the particular style is not of importance (Cartoon, Pixel Art, Realistic), shading is of importance. It should be barely perceptible, and closer to a gradient than realistic lighting.
Characters should be lighter at their centre, and darker at their edges.
Bosses should be lighter on their side nearest the vertical screen edge (Left/Right), and darker on their far side.
Backgrounds should be lightest at the bottom, and darkest at the top.
A suggestion is to decrease the lightness by 1% as you go across, and increase saturation by 1% for every 2% you decrease lightness (This is using HSL).
UI elements should have an Orange (#CC5800 to be precise) backgrounds, with a highlighting of Yellow (#E5BF00) and a shadow on the underside of the Orange, but above the Yellow (#662A00, with 50% opacity works).