Adding new buttons not working

I’m trying to add Flipboard and Yummly buttons using the sample Tumblr code from the FAQ. It’s working, except that the icons don’t show on the share buttons. They’re just blank. My theme didn’t have an assets/svg folder, so I created one and put them in there, but that’s not working. I also put them inside a folder in your plugin, but no luck there either. I even tried to put the full URL to the svg icon in the code snippet – again, no luck. Any help would be appreciated.