[EQAditu]

I'll use your third line as an example.
RStyleDefault="/WindowElements.DesktopWindowFrame.data.bkg.rect"
This is defined in Default\eq2ui_windowelements.xml (Line 68)

<RectangleStyle Center="center" CenterShrinkH="true" CenterShrinkV="true" East="e" EastShrink="true" Name="rect" North="n" NorthEast="ne" NorthShrink="true" NorthWest="nw" South="s" SouthEast="se" SouthShrink="true" SouthWest="sw" West="w" WestShrink="true" />

Each of the attributes points to an ImageStyle... such as "center". (Line 65-67)

<ImageStyle Filter="true" Name="center">
    <ImageFrame Name="image" Source="images/windowelements.dds" SourceRect="176,191,254,269" />
</ImageStyle>

You can define these styles directly in your own XML file and then change the RStyleDefault attribute of your background element to match.

So RStyleDefault is the default RectangleStyle that the element will use. A RectangleStyle can consist of one or more ImageStyles. You'll at least want to include a center ImageStyle for your RectangleStyle. Defining each of the eight outer edges to something different is up to you. An ImageStyle has an ImageFrame child element which defines the image to use and the part of the image to use. (Source and SourceRect)

A custom RectangleStyle can be within your root Page element and the ImageStyles should be peer elements.