JavaScript libraries (e.g. jQuery, MooTools and YUI) and ready-to-use CSS frameworks (e.g. Bootstrap, skeleton, gumby and foundation) are excellent tools for building websites quickly because they meet all the most common requirements. On the other hand, we only use a small part of their capabilities, yet these frameworks and libraries are not based on a modular architecture, forcing the user to download the whole library in order to use only a small percentage of its features.
Whenever possible, it is best to do without these libraries (c.f. http://youmightnotneedjquery.com) or to only keep the parts which are actually used (c.f. http://getbootstrap.com/customize). If this is not feasible, download the source code and create a custom build that only includes the necessary components.

Use a group of CSS elements instead of only one, and only call CSS that is useful for the situation. Doing so minimizes the size of the page when it is first loaded, thus saving bandwidth and reducing CPU load.