I would like to change the default font. Normally I would expect to find this in the Preferences but Preferences is grayed out.
Is this feature available yet? If so, where might I find it.
There are no preferences available in XD (yet), but feel free to enter a feature request here: Adobe XD: Feature Requests: Top (59 ideas) – Adobe XD Feedback : Feature Requests & Bugs
Regarding setting a default font, one shortcut you may find helpful is to select a text object which has the font you want to use, and then select the Text tool, click, and create a new text object where you want it. The new text object will have the same style (font, size, color, etc.) of the text object you selected previously.