You can add custom scss, css and js files inside Joomla Helix Ultimate 2 template.
1.For the scss files:
Add your custom.scss files here
../templates/shaper_helixultimate/scss/..
It will be compiled and then you can see the compliled-custom.css version inside
../templates/shaper_helixultimate/css/..
2. For the css files:
Add your custom.css files here
../templates/shaper_helixultimate/css/..
2. For the js files:
Add your custom.js files here:
../templates/shaper_helixultimate/ js/..