We are still unable to rename project folders in webflow. It will let you type the new name, but it will not save or rename the folder. This has been the behavior for at least the last two months. The only option is to move all themes into a new folder with the name you want. This is fine for folders with limited project items, but becomes very annoying for any folder with more projects.
As of January 29, 2022 this still appears to be broken.