Changing a repeater box background color based on what URL source is in the video player

I’m getting this error:

Property ‘at’ does not exist on type ‘$w’.

The other problem I see with this that I’ve been struggling with, is that of course I can get the box in the repeater to change the background color with a simple onClick, but once the color is changed, it doesn’t change back like the Wix Learn example.

If anybody knows who worked on the Wix Learn pages, it would be extremely helpful for me to get in touch with them, becasue those are the exact features I’m looking for in order to launch my courses page!