A few of my favourite themes for S60:
1. SiMo QVGA Theme
Straightforward, simple theme with a near-perfect combination of blue and black giving it a professional feel. Contrast between fonts and background colors is great and it is easily readable in sunlight.
Application size after Install: 2Mb
Designer: Zach Epstein
Download Link
Screenshots:
2. Frog
Visually stunning theme [...]