Repeating background image in sey design mirage

Hi, I want to change the repeating background image in the Mirage theme.
I was certain this was possible by alt click on the theme in the theme drawer and that’d show the file contents, and i simply needed to replace one of the stock backgrounds with my own…. but no luck.
Am I dreaming or is it possible and if so, how?
Im still on 5.3.2

I don’t have 5.3.2 any more but on the version of Mirage I have here, the ability to change the background repeating image is built into theme options. Looks like there are 10 built-in patterns and then space for 10 of your own.

Thanks - same on this version of the theme with space for 10 of my own - the trouble is I cannot figure out how to implement that - or find any documentation on the website to help.
I wonder if its a matter of naming the pattern i want as that of one of the options and dragging it into the resources section? I’ll try that.

Maybe this will help?