The Hub "theme" menu is straight-forward and works well.
The general theme and styling in Hub is pretty good.
Has the Hub team ruled out allowing Hub site creators to use custom CSS classes?
What I imagine would work is that in the theme menu there would be a section at the bottom where a button would add new classes to a list of custom class definitions. There would be standard form fields to edit. The value, instead of being a strict color, would be any valid CSS enclosed in brackets.
That is a very interesting idea and I will certainly make note of it. We are actively considering other methods for our customers to use in order to theme their sites. We're trying to achieve the right balance of customization, while also avoiding the mayhem caused by earlier versions of our software which allowed global stylesheets. (When we had global stylesheets, releases were often very stressful since customers' sites often broke because new features would collide with customized CSS in ways that we couldn't even being to guess ahead of time.)
We are also looking at color pickers that tie into a set of specific sub-classes (so you can reuse them for background, fill, text color, and border color.) Given that we currently allow you make custom classes through embedded style tags in a Text Card as you request, may I ask your goal? Is it that those classes aren't visible enough and you fear someone may inadvertently delete the card? Or that you want others to reuse them and they aren't visible enough in the Text Card?