Cockatrice is an open-source multiplatform supported program for playing tabletop card games over a network.
The program's server design prevents any kind of client modifications to gain an unfair advantage in a game.
The client also has a built in single-player mode where you can brew without being connected to a server.
Download an installation file from the sidebar on the left
Install 'Cockatrice' the same as any other application. (on macOS you need to drag 'Oracle' to your Applications folder too)
Open 'Cockatrice' and run the 'Oracle' tool to create a card database when prompted on first launch