One of the best ways to learn and appreciate CSS and web design is to take what is already out there and tweak it. Altering the CSS of existing sites has been easy for the last few years with tools like the Web Developer and Firebug extensions, or Chrome Developer Tools. What is new is the ability to share the results with others in the context of social media. Sharing visual remixes of sites not only demonstrates your own prowess, but also makes poorly designed sites easier to use for others.

There are a few centres for site theme sharing.  Those I would recommend include:

Stylebot SocialStylebot
A CSS sharing site that works in conjunction with the Chrome Stylebot plugin, allowing users to easily install site themes. Submitted themes are voted and commented on. Custom CSS can also be installed via user scripts.
Userstyles.org
Like Stylebot, uses a free extension (“Stylish”, available for Firefox and Chrome) to manage site themes. One of the oldest social CSS sites, with an active forum.
CSSPivotCSSPivot

Takes a different approach to skinning sites: no plugins or extensions are required. Instead, customization is built within the CSSPivot web application itself, and shared through a custom URL (similar to URL shorteners such as Bit.ly).