I'm not a developer either but you really don't need to be - at least not to make some very basic changes.
There are good resources on the forums about the userstyle and userchrome CSS that is worth having a look through. You can even have a look at themes that others have made and see how they did it.
- The CSS wiki is good for general info about how to do some specific things.
- The Share your CSS thread has a lot of info about how to style certain areas as well as lots of examples of themes people have made.
If you have any particular questions people are usually willing to help out.