Mailman lets you change the format of some pages, especially the templates introducing individual lists but many pages, including the master lists page and the list admin pages, cannot be tweaked at all. They don't even use style sheets, so you can't apply a custom style sheet (which would seem like the obvious way to allow administrators some global control of page content).
To get around this and allow us the opportunity to put our own style on these pages any page which shows the word "(modified)" at the bottom has been passed through an external filter by Apache before being served. This filter inserts a <link> tag to an external style sheet, strips out some of the more primitive mark up used by mailman and replaces it with the use of "class" attributes.
If you would like a copy of this filter please drop me a line.