Found under: Settings > Song Actions > Song tags

Tags can be used to group songs into various categories, themes, song books, etc.   In the desktop version of OpenSong, tags are referred to as 'themes'.

You can edit individual song tags from the Edit song page, but if you wish to bulk assign tags to multiple songs at once, you should use this option instead.

When displaying songs in the song index, filter by tags is the furthest right option at the top of the screen.

When editing a song via the Edit song page, you can assign an existing tag to the song.

If you want to add a new tag, expand the popup window, or scroll down and enter a new song tag name.

Checking or unchecking a song tag item will add or remove that tag from the existing song.

Clicking on the trash icon will remove that tag from all matching song, effectively removing the tag.  You will be asked to confirm this action before proceeding.