Is it possible to change the icon of folders in the side panel in Files?

Welcome to Fedora @ketseuki

I guess it is not possible. This icons depend on the Theme you are using.
And in Adwaita for example, the link Coding points to a folder.

If you right click in the side panel on properties, you have the possibilities to change the properties. This just changes the explicit icon of the folder. The Folders in the left panel are mime-types ( as I suspect). If you change the folder Icon there, all folders will have this Icon.

oh … got it. Thanks for the reply!