I have low vision, and have enthusiastically embraced the idea of using userChrome.css to increase font size (especially the message list) that Theme Font and Size changer doesn't address. I wrote the following css, but only the background color portion works. Where have I gone wrong? @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* Folderpane(color/text) & Messagepane(color) */ treechildren { background-color: #fffff0 !important; font-family: Segoe UI !important; font-size: 20px !important; }