Change CSS values with WordPress backend UI elements

I have seen in most premium WordPress themes, backend user is allowed to change CSS properties such as background-color, font-color, font-family, just withing the backend using certain UI elements.

There's a color picker on the Appearance section and user can change the color of the particular element and save it.

I'm building a small wordpress theme, and I'd like my users also could change CSS values such way.

So, what's the way of doing it ?

Is it something like file writing ? Or dynamic ? (storing CSS in the database? )

`Thanks!

via Chebli Mohamed