From Documentation
Some container component such as window, groupbox, detail have a content block, you have to use contentStyle to set its style.
For example,
<window title="below is content" contentStyle="background:yellow">
Hello, World!
</window>
Scrollable Window
A typical use of contentStyle is to make a window scrollable as follows.
<window title="Scroll Example" width="150px" height="100px" contentStyle="overflow:auto" > This is a long line to spread over several lines, and more content to display. Finally, the scrollbar becomes visible. This is another line. </window>