What do you mean with "Fluent icon pack"?
OC can use either Windows Default under Folder icons or you can use custom ones.
In \Resources\FolderIcons you will see two folders and png files in there. If you have some other icons as png files, you can create another folder there with any name (example "MyIcons") and put icons there. Icons should be called the same name as folders, case sensitive, so Music.png will replace folder "Music" but not "music". If you add folder called "asdf" in sidebar, and you have "asdf.png" it will be loaded. If you prepare your own icons, it is best to keep them small, 32x32px.
Then you can open settings and in Folder Icons folder select the "MyIcons" folder, or however you have called it, then close settings and restart the program