How do I sync my icons on multiple devices?

In the Settings page within the Mac/Windows app (accessible by clicking Account > Settings or pressing ⌘+, on Mac or Ctrl+, on Windows), you can change your library location.

If you want to use the same icon library on two devices, you can select a Dropbox/Box/OneDrive/Google Drive/Nutstore folder. Please note that this feature was not developed for working simultaneously on multiple devices, but just to allow the use of the same library.

If you select a folder in the cloud to store your library, do not use the app at the same time on multiple devices. If you do, Dropbox/Box/OneDrive/Google Drive/Nutstore will create a copy of the database (conflict), which will cause the icons to disappear from the app.