get Back Color
Gets the default background color for the template.
This color is typically used as the base background for areas of the template not covered by other content or modules. The color is represented as a string, often in a web hex format (e.g., "#RRGGBB" or "#AARRGGBB").
Return
A String representing the background color, or null
if no specific background color is defined (in which case a system default or transparent background might be assumed).