durandal.css
durandal.CSS comes with Durandal.js and controls minor layout/appearance for elements within Durandal. This variable works mostly to do with the modal dialogs (those that host the portal’s message inbox or select meter dialog).
Using durandal.css
Do not alter this variable unless it is necessary and you are an experienced CSS3 programmer. There are overrides in the durandal overrides section of app.less that give control over the dialogs.