Transferring unused CSS is a common problem, and adds weight to the page or more resources to download before rendering (rendering is blocked until all CSS has been retrieved and parsed).

CSS selectors have different performance, though it's negligible on most users devices today. One common misunderstanding is the way CSS selector are processed. They are processed from right to left, so a "#mystuff div" will actually lookup all div's in the page and check if it's a child of #mystuff.

I never use any frameworks, they are not worth the hassle. Just start from zero (expecting you actually know enough CSS). Typically, when I write CSS from the ground up, it's just a few kilobytes without any unnecessary code. Using more targeted CSS (I try and keep everything referring to ID's or specific classes) you will also end up with less HTML (no "button button-big button-blue button-shadow" classes all over the place).