If you like some of the seven Nokia 5250 original themes for your Nokia N97 or N97 Mini, you are lucky. Shedme666 extracted the seven original themes, so that we can install in our Nokia device our theme favorite.
Attention to the black theme, impossible to save more battery power with a theme 
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Download | Nokia 5250 original themes
“impossible to save more battery power with a theme”
Actually as far as I know, it is possible on certain OLED-display phones such as the N85, but not on LCD phones such as N97, 5800 etc.
OLED: white takes power, black does not (black theme saves some power although not very significant unless the phone is used very frequently).
LCD: any color takes roughly same power (black theme has no effect)