I was going to do a Cowboy Physics joke and did some research to find out The Right Way (what the exact verbiage is, what background to use) and noticed that some of the older strips in the series use "background2" which isn't assigned to a set.

 

So, uh, how does one make use of that one if it can't be selectedfrom the dropdown?  Also noticed (cuz I'm nosy) there's a background3 but it doesn't seem as useful, blue on blue...

 

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Say something cryptic then leave snickering.