Normally with themes it would be time prohibitive to do so, but the nature of utility-css is that it's relatively straightforward to implement each component, even without looking at the CSS that's obviously easy to inspect.
I think what they should do is build a tool that let's you build your entire site using these components and charge for that. Otherwise, I guarantee within a year of today, 2/26/20 there will be a free version of this on GitHub created by the community.
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On a side note, if some bored person out there wants to make a huge splash in the CSS community, figuring out a way to target a specific DOM element, create the equivalent of an AST and specify a "dictionary", which would be a utility-css framework (tailwind, tachyons, basecss, etc) and finally reimplement the targeted DOM element in the chosen framework would be amazing.
Assuming you buy into utility-css, that would remove all existing friction in adopting it. Then that would mean you could grab an existing theme [1] and convert it to tailwind components as desired.