To manually install a theme, open a terminal, change directory into ~/.themes and unpack the theme (under ~/.themes).
For light-on-dark, my top picks are
gm-looks By elgraz (3.0 kb)
marble-ice By floe (1.4 kb)
BlueHeart By JP (94.2 kb)
For dark-on-light, my top pick is
Clearlooks9x By netdur (1.3 kb)
Default "blueish" color variant for cillop gtk theme. Required pixmap and clearlooks engines.
The following Dark Background GTK Themes have promising thumbnails, but they don't look good on my CRT (my normal display brightness in 18~24).
documented on: 2007.01.01