Allows administrators to inject CSS into the page output based on configurable rules. It's useful for adding simple CSS tweaks without modifying a site's official theme -- for example, a 'nighttime' color scheme could be added during certain hours. The CSS is added using Drupal's standard drupal_add_css() function and respects page caching, etc.
This module is definitely not a replacement for full-fledged theming, but it provides site administrators with a quick and easy way of tweaking things without diving into full-fledged theme hacking.
Please note: Creative individuals can use CSS to execute code on a client's browser, by injecting JS snippets in place of images and so on. Treat the 'Administer CSS Injector' permission carefully, as you would any other sensitive administrative tool; allowing untrusted individuals to use it is a security risk.