This Simpler CSS plugin allows WordPress µ hosts to enable users to add custom stylesheets to their blogs.
The plugin creates a new menu item under the Appearance menu that allows users to enter custom CSS code that will be injected into the section of their blog. The CSS is stored in the blog’s options table and is filtered through a standard PHP function before being outputted, preventing the user from inserting malicious code into the header.
For non-WordPress µ blogs, this plugin provides an easy way to modify the appearance of installed themes or plugins (such as Sociable) without modifying theme/plugin files that may change with upgrades. No write access is required to any files for Simpler CSS to function, as it stores its data in the database’s options table — and that means theme/plugin upgrades won’t impact your custom CSS.