You can find the files that are used to build the frontend vue app in the /vendor/laravel/horizon/resources folder.
The page templates are part of the vue components (.vue files) that are found within the assets/js/pages folder. The css is built from sass files found in assets/sass.
You may be able to make superficial changes to the above files without any vue knowledge, however doing some reading up on how vue components are structured and other vue basics will definitely help.
After you make changes you will need to build the app.
There are build scripts included in the horizon package (check the package.json file).
These scripts will update the files that are included in public/vendor/horizon with the built files.
It would be up to you how you choose to track the changes you make, but I would avoid editing the files within /vendor/laravel/horizon directly, but rather fork it and import it in as your own package, or edit and build the frontend files separately to include within your project.