in the status bar, choose Write New Style, and copy the CSS from the style's page into that dialog. Press save and the style is installed.Installing styles from this site is as easy as a pressing the "Load into Stylish" button on a style's page when you have Stylish installed. A dialog will come up where you can rename, edit, and preview the style. Once you're happy with the style, press Save. The changes will take effect immediately.
Global and site styles can also be installed with GreaseMonkey. Click on the "Load as User Script" button on the style page, and GreaseMonkey will take over.
To install a style into Thunderbird when you have Stylish installed, click on the in the status bar, choose Write New Style, and copy the CSS from the style's page into that dialog. Press save and the style is installed.
All Mozilla applications support user styles by means of userContent.css (for site and global styles) and userChrome.css (for application styles). Read about their use at the mozillaZine Knowledge Base. You may have to remove the @namespace line at the start of the file, if present.
Opera's UserJS feature allows the installation of global and site styles. Opera does not support app styles. Clicking the "Load as User Script" button on a style will provide you with a version of the style formatted for use with this feature. For information on how to use UserJS, see Opera's documention.
Styles may also be installed using Opera's user stylesheet feature. Most styles will require editing to make them compatible with this feature (e.g. removing the @namespace and @moz-doc-rule lines).