2FAS is an interesting app as it focusses more on privacy than Google and Microsoft’s 2FA authenticators do (we all know Google and Microsoft love to know where you log in, from where, and when). To this end, the app operates on its own and, if you choose to, it syncs between devices using your own iCloud or Google Drive. It requires NO account registration to be used.
It has a dark mode, as well as the ability to group your 2FA tokens, and can also show the upcoming 2FA token (useful if there is say 15 seconds to go, and you don’t want to wait). It is compatible with any service that supports the TOTP and HOTP standard, including Google, Microsoft, and Dropbox.
There are two potential downsides right now: Firstly, this works with one or more mobile devices, so the desktop browser extension does not run its own tokens (it calls the mobile device for an OK). Secondly, this could be a problem if you use an Android as well as an iOS device, as there is no syncing between the iCloud and Google Drive storages. However, migrating from one OS to the other should not be a problem as the app can export and import the tokens.
If you want to work offline without the cloud sync, just remember to make a copy of the backup codes or save (with a password) the tokens to a file, and move that file off your mobile device.
Their code is open-source, including the server side, which can install using a Docker image.