If you use Fisher to manage your fish plugins:
fisher install dracula/fish
If you use Oh My Fish to manage your fish plugins:
omf install https://github.com/dracula/fish
If you're using fish >= 3.4.0, you can set the theme by adding this line to the top of your config.fish:
fish_config theme choose "Dracula Official"
If you're using fish < 3.4.0, then the theme is set just be installing this plugin. Note that this means that you can't install this theme alongside others and switch between them freely, which is the main advantage of using fish_config.
Download using the GitHub .zip download option.
If you're using fish >= 3.4.0, place themes/Dracula Official.theme in your ~/.config/fish/themes directory. You can set the theme by adding this line to the top of your config.fish:
fish_config theme choose "Dracula Official"
If you're using fish < 3.4.0, place conf.d/dracula.fish in your ~/.config/fish/conf.d directory, then reload fish (e.g. exec fish).