To allow border images to be reused on different backgrounds, and thereby reduce the number of unique images required for a theme, transparent PNGs can now be used instead of opaque images. For the transparency to work, the images may no longer overlap at runtime. To achieve this, the theme must explicitly specify the size of the used images, so that they can be correctly positioned.

For every background type (cpheader, cpbody and cpbottom), a new CSS class ending with -size must be defined (i. e. cpheader-size, cpbody-size and cpbottom-size), similar to the already existing classes in css/bgimages.css (e. g. cpheader-l, cpheader-tl, etc.) The new class must specify the width of the used images in the properties left, right and width, and the height of the used images in the properties top, bottom and height. E. g.:

.cpbottom-size {
left: 12px;
right: 12px;
width: 12px;
top: 20px;
bottom: 20px;
height: 20px;

Since all themes must add the new classes, this is a backwards-incompatible change.