UserTags is a database of custom Discord tags made entirely via CSS for Discord theme devs to use in their themes.
Getting your own tag can be done by submitting an issue with your tag information (your Discord ID, your tag text, and your tag color) or creating a pull request with your tag added.
I will most likely open a Discord server for tag requests if this becomes big enough.
If you'd like to include UserTags in your theme there are two ways to do it:
1. Import the basic implementation. (SIMPLE)
Adding tags from UserTags without making your own implementation is as simple as adding @import url('https://cr3atable.github.io/UserTags/basic.css') to the top of your theme.
2. Build your own implementation with the DB.
Building your own implementation is a good way to add custom tags if you have a theme that makes a lot of major changes to Discord's UI (ex: Tropical's Midnight).
You'll want to add @import url(https://cr3atable.github.io/UserTags/tags.css) to the top of your theme which will import the database (note that this does not add custom tags by default).
I'd suggest looking at https://cr3atable.github.io/UserTags/basic.css to get a general idea of how to make your own tag implementation.