When I put a TreeView or MonthCalendar in a MenuTool, the popup has fat, ugly two-tone yellow borders. Which style do I need to mess with to get rid of those?
An element put into a MenuTool's Items will be hosted within a ToolMenuItem so you could explicitly put it into a ToolMenuItem whose Template is set to a custom control template that just has a ContentPresenter. To just alter the colors, you can look at the default xaml for the ToolMenuItem - the default xaml is provided in the DefaultStyles directory.