<bg_color>

The background color for a text banner or a title, represented by a hexadecimal code preceded by 0x.

To find the hexadecimal code for the color you want, use a color picker such as this one.
Child of: <text_banner name="">, <title>, <title_defaults>
Parent of: Nothing

Example:

<bg_color>0xDDDDDD</bg_color>

Working example: Search for <bg_color></bg_color> in the file trivia_demo.xml.