Not every tweak or theme is automatically available once you jailbreak your iOS device and install the Cydia application. For certain tweaks, you'll need to manually add its source, which is simply a web address linked to repository where the tweaks are hosted.
Generally speaking, if a tweak isn't hosted on one of the preinstalled sources on Cydia, like the BigBoss repo, then the developer will provide you with their source. For example, elijahandandrew.com/repo/.
Once you've got the developer's source link, enter the Cydia app and navigate to Sources -> Edit -> Add. When prompted to, enter the source link and tap Add Source.
With the source now added, you can enter it to scroll through the packages, or use the search option found in Cydia to locate the tweak hosted by the new source.
Have an iPhone? Check out all 200+ new features coming in iOS 13.