@klogg, here is a link to arsaboo’s card: Custom animated weather card for Lovelace. The instructions there have links to his original files as well as the animated icons that you will need. I would recommend getting that to work first and then if you are still interested in using Dark Sky as the weather source, modify it using the code in my post as an example.
To answer your other question, the .js and .css files go in the /config/www/custom_ui directory. If this is the first custom card you are using you may need to create the www and custom_ui directories.
Note that the css snippet in my original post only shows a small piece of that file (just the change I made). The change is not really necessary so using the original css file works just fine. I renamed it in my configuration. You can see the reference to it in the dark-sky-weather-card.js code (including its directory).
At the top of my ui-lovelace file I have the following:
resources:
- url: /local/custom_ui/dark-sky-weather-card.js?v=4
type: jsThis tells the UI where to find the custom card. Note that the url is listed as /local/custom_ui/dark-sky-weather-card.js. The /local directory refers to the config/www directory.