userstyles.org (the site you're on, if you haven't noticed) is the largest collection of user styles.
You can search terms in the search box to find styles. You can also enter a URL - the site will return all styles that may affect the site at URL.
Another way to find styles is to browse the categories on the site. Site styles affect only certain web sites. Global styles affect all web sites. App styles affect the user interface of Mozilla-based programs. These categories may have subcategories to further refine your search.
Stylish is the recommended way to install styles. There are a few other ways.
Some styles also available as user scripts for use with Greasemonkey. These styles have a "Load as user script" button on their pages.
To post new styles on userstyles.org, you will have to create a user account. Once logged in, you'll see a Create new style link on your user home page.
Styles are limited in a few ways:
userstyles.org will categorize styles based on the code they contain. It will classify styles as Site styles (styles that affect a certain site), Global styles (styles that affect all sites), or App styles (styles that change the user interface of Mozilla-based applications). It may also subcategorize styles within those categories.