It has a checkerboard pattern because it's a textured background with a transparent yellow tile over it, on a screenshot from JCS

There's two separate textured backgrounds available in the set plus a few single-color tiles that can be used for overlays, so it should easily be possible to make a background like that in-game too.

Violet: thanks, but that might be a bit too much credit for me. While I've left very few tiles untouched, most of this is still taken from various Waz sets. The set began as an updated version of Waz 11, but though the bulk of the set is still from there I've included some things from his other sets (for example the curtains) too.