Restorator is a very good program it makes editing system files very easy. I realize it's not a cheap program but I sent them an e-mail before I bought it requesting a discount and they sold me a student license which was at least 30% off.

If you are going to make custom themes editing the .msstyles I would recommend purchasing Ave's Windows7 Style Builder there is no trial for this but it is only about $22.50 US and it is a great program.
Jerry