Not understanding kubrick’s side borders are created

OK, so I can see that the sidebars are created by putting kubrickbg.jpg in the background and stacking it vertically so that the borders can stretch as necessary to contain the content. The problem is I can’t locate where kubrickbg.jpg is called for. I can’t find it mentioned in any of the php or css files, but when I open the index.php in a browser and view the source I see this bit of code near the top:

#page { background: url(“https://the1-2-3-4.com/development/wp-content/themes/the1234/images/kubrickbg.jpg”) repeat-y top; border: none; }

Can someone explain how this instruction is generated by the php files?