CSS is for styling, there's no such thing as back-end CSS, try PHP.
Time spent coding, quality and amount of code are more serious, objective, verifiable and overall professional values to measure the value of this kind of work.
Your "not satisfied, not paying" policy won't attract any quality self-respecting designer/coder unless you couple it with an advance and/or some other kind of bonus(es).
Also, the fact that you think that you can use CSS to code an admin panel plainly reflects that you yourself don't even know what you want.
Just saying.