Most of the Aftermarket rims especialy the high performance wheels are lighter than the stock. If you get the same size wheel and tires, you dont have to worry about anything. If you do get the larger ones, use the link below to calculate what size.

http://www.miata.net/garage/tirecalc.html

I have 17s on my car, a size bigger than the stock and they are gunmetal grey. You can look for something similar if you like. they will look better when you lower your car.