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.
Preferences are still an outstanding issue (2 years later!), but a better way is available now to make font styles for your look.
Format a text object for your "default" font characteristics, including font, weight, color, and size. In the left menu, under Character Styles, click the + sign. It will create a character style for your preferred "default" style.
Then you can create any text and with that text object or the text itself selected, click the Character Style you created, and it will be reformatted to your preferred style.
You can do with with multiple text objects at once.