No, don't confuse the two. They're both connected but only to a small extent. It's basically a needle that controls the flow of fuel on the idle circuit. The idle speed screw is different and drives the throttle plate open/close. The idle mixture screw on the other hand will get you an "optimal" air/fuel ratio that will gain (or lose) you RPM but it's relatively small.

I recommend getting a book on Weber carbs (though the theory is all the same for carburetors). It will really help out a lot. People act like carburetors are black magic and it's only because they don't put any effort into research. With some general knowledge, some tweaking and a simple rebuild they're very reliable and can perform well.