How to customize the variant selection menu

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I am using the Handy theme by Pixel Union and I am trying to customize the bottom "add to cart"/variant selection area. I know some html, css, and javascript but can't seem to find any direction on how to begin doing this. 

I found this website I like that customized their bottom menu/variant selection and I was wondering how I can customize mine like this -

Any direction or help with this would be greatly appreciated.