@Teemberland thought as much. I personally don’t use Slick for anything because of the issues it has. However, it’s abundantly clear that this theme doesn’t seem to recognize or inline any css from the ACP which is bizarre.
The good thing is that there’s nothing wrong with your NodeBB installation, but as you say, you have to use Slick, so the only way this will work is to embed the CSS in a widget. Whilst this works, here be dragons. If you decide to change the theme or have to reset it, the widgets are all removed and placed into the drafts section - you have to put these back manually.
My last remaining suggestion would be to minify the CSS once you have it finalized as it’s presence in the widget could well affect page load speed.