Welcome, and before you get flamed massively for not using the search function first and foremost, as I did one time back in the day....I will start you off in the right direction.
First off, our colleagues at GT (GeekTekneek) Have a Youtube channel which can show you a brief idea on how to customize themes and use the current home launchers to do it. That is this link below :
http://www.youtube.com/user/pluggedin2010
Use that, I know that they are working on more tutorials over the next week for all the community users.
2ndly....Click on search the forums and type in the following - Windows 7 Theme, paulmz , launcherpro . This will give you some results on the themes that are out there and what they may look like on your device. But for the how to's either look at GT's youtube channel, or look on xda developers forum also.
There are tons of how-to's on here also, just search for them. However, since I and my company at FluxboxMedia works closely with GeekTekneek Team, I know their tutorials and help are great, and they answer their emails and such really quick. If you have questions, their team will help you to the best of their ability.
Hope this helps, good luck and enjoy. You will absolutely love your phone!
Customizing Video #3
http://www.youtube.com/watch?v=HKtSDmndD7c
Customizing Video #2
http://www.youtube.com/watch?v=xXQ_betXjQU
Theming Video #1
http://www.youtube.com/watch?v=Uh4RBMz1ZwQ
-FBM Administrator and Owner