It looks like it renders the default theme (white) for a couple of frames before showing the current theme.
It only happens when I click DO on taskbar to call it up again after I minimized DO manually. If DO is just hidden by other programs (maximized), and then I click the DO icon on the tasker, DO shows up normally, it shows the dark theme immediately, no white frames in between.
If you cannot reproduce this case for the first time, try to switch off the light, or use a dark wallpaper.
For the dark theme in this topic, it is
Simple Windows 10 Dark Theme.
And it is DO 12 light.