I have installed Extension_CSS which allows the user to create custom CSS styles and save them as a wiki page. They can then be easily applied to an article anywhere on the site.
If you do not know what CSS means here is a Brief Tutorial
Click on Extension_CSS for a description of how to use this.
I suggest that we adopt a consistent naming pattern to avoid confusion. I would suggest that all style pages be named in the following manner: 'Style_name.css' replace 'name' with the name of your style. Then place a link to your page here along with a description of what it does.
As editing a style page will change the appearance of every page that uses that style, it would be polite to discuss it before doing it, or save your new style with a different name.