Yes, the background changes instantly, but video replacement is slower, therefore for an moment the old video shadow background appears distinctly from the site's background color. This is on M1 Max 64 with latest Chrome.
Also, there's a problem with responsive text reflow on your personal site, I've only noticed because I'm using a tiling wm.
I think for the first issue, with the flashing, it seems to flash to you, but it's just the theme being changed to light, isn't it? Or you are specifically only talking about the way the video container doesn't fit the rest of the page while it's loading?
Regarding the second issue, good catch! I'll fix that. Thanks!
Ha, I think you're right, it's the empty container with grey background visible whilst the video is being changed, I actually went frame by frame to confirm. It's getting silly and super nitpicky so feel free to ignore :)
Also, there's a problem with responsive text reflow on your personal site, I've only noticed because I'm using a tiling wm.
I've made videos to showcase
https://gofile.me/7cmOh/e1dTClPiT
https://gofile.me/7cmOh/VyHzagGGa