Ah, thanks. That css url goes to /assets, but on your wagn it should be /wagn/assets. You can fix it in the card named "style: traditional". Will fix in code as well.
Is this something specific about my wagn setup? Also, I assume I have to reload this style to make it show up. How do I do that?
Wagn 1.12 introduced some issues into "non-root-path" wagns, meaning those where wagn is installed in a subfolder (in your case '/wagn/'). This is common in cloudstore wagns, because cloudstore makes "/wagn" the default subfolder in order to make it easy to install multiple apps on the same site.
to reload the style you have to re-save the *style rule. see the new and improved skins docs, which cover this more clearly now.