Here are two great themes Green Board and Deep Blue for Nokia 5800 XpressMusic from MMMOOO. These themes are well designed and fully compatible with Nokia 5800 and Nokia N97.
Green Board
Green Board is a free theme from MMMOOO. It's a theme with Royal green board bar and pure black background. This theme comes in default icons and custom icons versions. You can read more about this theme and download it from MMMOOO.
Deep Blue
Deep Blue theme for Nokia 5800 XpressMusic and Nokia N97 is again a free theme from MMMOOO. It's a full SVG theme. You can read more about this theme and download it from MMMOOO