I like the icons and I use them a lot. It would be nice if we could change the colors.
Additionally, here are some suggestions for more icons: more social media icons like Instagram and LinkedIn. A keyboard, table, chair, desk and just an overall update to the icons, but definitely increase the selection.
I myself love prezi icons as well. Thanks for the suggestion I will write this to our list of requests.
Meantime I recommend that you go to Insert > Image > Google search bar - you can easily look for Instagram logo png , Linkedin logo png. usually when you type png, you will find images with transparent background.
Can you also let me know for what kind of presentations do you use prezi ?