Currently there seems to be no way to open the options of dropdown commands like "Font Family" or "Font Size".
The typical hotkeys of the according Kendo UI widgets (ALT + Arrow down) are not working and we have not found any other.

This can be reproduced in its online demo:
https://demos.telerik.com/kendo-ui/editor/keyboard-navigation?_gl=1*1jjk3dq*_gcl_au*NjE0NTE0MTQ0LjE3MjYwNTc4MjU.*_ga*MTk3MTcyODQ3My4xNjkwMjkyNTYw*_ga_9JSNBCSF54*MTcyOTY3MzExMS44My4xLjE3Mjk2NzQwMzAuMzEuMC4w