Thread:Merrystar/@comment-4146635-20120311161910/@comment-11001-20120313030819

He means to add the lines of code he gave you to the page Mediawiki:Common.css on your wiki and click to publish it. The "2px" is how wide the border around the thumbnails will be -- just play with the number until you like it. The "solid #000;" is the color (black in this case). The link he gave on "colors" is a web guide with codes for a lot of colors so you can pick the one you want. Some basic color names are also recognized (eg. "solid red;" would work).

Once you like how it looks all images displayed as thumbnails on the site will have the border.