Steps to reproduce:
Set custom new tab page to something with a dark background
Open new tab, see white flash before page loads
I checked the safari behavior, and if you have light mode enabled the loading screen is white. In dark mode it is black. I would suggest using that same behavior.
Orion, OS version; hardware type:
MacBook Air (M1, 2020)
Orion Version 0.99.115-beta (WebKit 614.1.12)
Happy to record one if needed, but reproduction is pretty straight forward.
(Fwiw, so far I am loving Orion and see myself switching from Safari 🙂)