I'm a frontend web designer too

I thought about having a CMS made that I could easily skin and sell to customers, but it depends on how many customers you expect to sell it to. A good CMS wont be cheap, so you have to make sure you can cover your costs.

The other option you could go for is an open source CMS. While you couldn't charge for the actual CMS, you can charge for installation, setup and the skin, plus any extra work you do.