I get the same issue with undefined
Seems like _variables.css is not being correctly imported on build
Also minified css turns out to be identical to original one

Yeah, I’m having the same exact issue.

It looks like the tachyons-build-css tool is referencing an older version of postcss-css-variables:

The most recent version of postcss-css-variables is 0.17.0.

Troubleshooting

I also did some troubleshooting and when you add a fallback value to the CSS-variable, the fallback appears in the ouput. Here’s an example:

_spacing.css before compiling:

.pa0 { padding: var(--spacing-none); }
.pa1 { padding: var(--spacing-extra-small, 0.25rem); }
.pa2 { padding: var(--spacing-small); }

tachyons.css after compiling:

.pa0 { padding: undefined; }
.pa1 { padding: 0.25rem; }
.pa2 { padding: undefined; }

This make me feel like this is an incorrect options issue in the tachyons build tool itself.

Hoping @mrmrs or @johno can chime in on this sometime soon :-)

Did some digging and figured out what the problem is and solved it in #16.