Elegant fonts, few colors, and a simple layout can make a significant difference.
They involve aesthetics (overall beauty), signs (specific meaning), form factor (specific geometry), and interface (elements of control).
Greyscale Drafting - Making a design with shades of grey first before choosing colors.
Principle of Few Colors - Good themes focus on 1, 2, or 3 colors at most. Too many colors distorts the viewer experience.