Is there Something like a Dark Background Excel Theme?

I would do as @CharlieRB suggests and additionally set a small background image, any color, or image that works for you. You can make this a default template as well if you like. Not sure how printing will work for you, but you could set a custom view to deal with printing, if that is an issue.

Page Layout>Background

Create and select a small image

Yes there is. This can be done in both Excel 2007 & 2010. I have 2010, so to change it go to

FILE > OPTIONS > GENERAL

These options are available there;

ok so i just figured out a solution, but it comes with drawbacks. you can use windows aero to change the default color of all programs which will do exactly what you're asking for, but will affect most standard programs as well. so here's what you do:

1. right click on desktop
2. select 'personalize'
3. select 'window color' button
4. select 'advanced appearance settings'
5. in the box that opens look for a box that says 'active window'
6. click in the white space of that box
7. just below said box it will have a drop-down menu titled 'Color 1'
8. select this drop-down menu and choose any color you'd like

This will change the background color of excel, word, etc. Hope this helps.