Familiarize yourself with these critical elements as a first step.
The LUCI library sketch file contains design elements and symbols such as color palette values, buttons, form elements, icons, etc.
A library is just an ordinary sketch document that contains that you can use in any other sketch document. If any of those are updated in the master library file, documents containing instances of those symbols will display a notification telling you that they can be updated.
Note: You should not make edits to the library file symbols directly, but should instead edit the symbols exclusively in your project document. You can do this by detaching symbols from the library or importing them as local symbols.
The zip file contains SVG and PNG formats of the LUCI icon set.
Need something else? Contact the LUCI team to make a request.