You are free to install this script/stylesheet for yourself, but it is not allowed to be used wiki-wide (e.g., in MediaWiki:ImportJS, MediaWiki:Common.js, MediaWiki:Common.css, MediaWiki:Fandomdesktop.js, MediaWiki:Fandomdesktop.css, or MediaWiki:FandomMobile.css), as it would violate Fandom's Terms of Use. (See the customization policy)
HydraRevived is a stylesheet and script that reintroduces elements from the legacy Hydra skin to FandomDesktop. Although it does not copy Hydra's design exactly, it reimplements the horizontal sidebar, page header (with search bar), and the Gamepedia-esque global navigation.
Note for admins: Although HydraRevived makes minimal changes to the article body area, it's best to disable this stylesheet when designing your wiki, as most other users will not be seeing the same layout as you do. You can design your wiki for the unmodified skin and make the changes you want to see with this stylesheet in your personal CSS.