I have figured it out!
I have to add !important at the end of this line (right before the simicolon).
body.custom-background { background-image: url('<?php echo $page_bg_image_url; ?>'); }
It has broken the alignment of my background image for some reason. Hopefully I can get that fixed now. I’ll probably have to do it with CSS.
EDIT: Never mind about the alignment, I accidentally changed it.
Ah! Thank you! I looked everywhere for a way to do that. I don’t know anything about PHP, so I guess I overlooked that echo.
Again, Thanks! 😀