@dvvdv if you still would like to know how to do this then here's a quick rundown:
Disclaimer: I have not in recent years used this type of Sidebar.
Disable SIP on your Mac. "csrutil disable"
Download mySIMBL from here on this page(there's a link a few scrolls down)
https://github.com/w0lfschild/mySIMBL
Then install the ColorfulSidebar SIMBL plugin from the repo within mySIMBL.
You can also download the latest XtraFinder app or TotalFinder(small amount of $$) and have the sidebar use colorful icons that way and not install mySIMBL. XtraFinder has returned after some hiatus and it's functioning fine on macOS Sierra.
Now obtain the Flurry System icons from the IconFactory:
https://iconfactory.com/freeware/preview/flrs
https://iconfactory.com/freeware/preview/flr1
https://iconfactory.com/freeware/preview/flr2
Download LiteIcon from here:
https://freemacsoft.net/liteicon/
Change your Home folders to that of Flurry - but leave out replacing all the harddisk icons - replace them individually. LiteIcon explains it better.
When you change your Home folder icons such as your Pictures folder - then the Sidebar plugin will/should calculate the smaller size from your icon (the 16x16px one) and apply that. I'm not sure though if you will ever get it as sharp as that in the the screenshot because the size of the sidebar icons in Sierra is a little bigger? per default.
If you use or have a license for the PathFinder program then that's also usable for a sidebar like that. I would not, however, recommend PathFinder 7 as the interface looks awfully dated and v8 of said program is coming sonn'ish(I don't know if that would ever have an impact)
This is the rundown that I would expect could help you along your way.