Hi,
The above code did not modify anything. Of note, that code is pointing to an .svg instead of an image file. I did find this file; {SITE}/templates/rythm/images/ticket-bg.png, which has the exact dimensions of 1608 x 218 used by the events background pop-up image. I renamed that file to ticket-bg.png.OLD and the background pop-up disappears. Oddly though, when I created my own 1608 x 218 ticket-bg.png and placed it there, the light grey image magically reappeared instead of the image that I created. I then tweaked your code to that shown below, moved the ticket-bg.png image that I created into the {SITE}/images/ folder, cleared the site cache, and then reopened the site. Instead of getting a pop-up, I now just have the ticket-bg.png constantly being displayed. Not sure where to go from here.
Regards.
.event-list-item:before{
background-image: url(../images/ticket-bg.png);
}