I have changed my tooltip font from the default. When toggling Dark Mode the tooltip font reverts back to the default font. Note that the font I set still remains selected in the settings, the actual tooltip doesn't use selected font when displayed. From that point to show the correct font requires restarting DOpus or resetting the font in settings (even though it's already set) by selecting another font and back to the desired font again.
Thanks! Fixed for the next beta.