It is already here the version 1.0 of the Dark theme for Nokia N900 created by MowmoTheme. It is still in development, but that we can see we have liked very much.
The theme is in development, v1.0, so that if you try it and see some bug, you can comment on it 
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To install the dark them you follow these steps:
I’m having problems installing this on my N900. I placed the folder in :/user/share/themes but it doesn’t show up on my phone when I click on select themes. any suggestions?
i found the solution to install this theme:
first you need to install ROOTSH on your n900, for that please refer on this site http://www.nokiadistrict.com/root-access-on-nokia-n900-with-rootsh/
second, you must install OPENSSH server on your n900 and WinSPC to your computer, for that refer on this site http://www.nokiadistrict.com/openssh-on-nokia-n900/
i’ve just shared what i’ve done already and finally, i have this theme now….
tnx for themes good