If Orion was already open when a system switch from dark- to light-mode (or vice versa) is triggered, the title bar does not correctly reload itself in the new color scheme. (Perhaps unrelated: If you keep browsing using Orion it does load the correct expected light/dark mode for websites though.)
Starting in dark mode with a normal title bar:

And then switching to light mode gives (with correct font colors, but should be light background not dark):

This issue only seems to occur if the switch from dark-to-light occurs while Orion is in fullscreen, is the currently being viewed program, and using the menu bar button to switch between theme modes on the Mac.
I would expect the title bar to reload itself in light mode, thus making the background the standard light gray with black text on top:

Version 0.99.137-beta (WebKit 623.1.8.0.0)
Tahoe (26)