Greasy Fork

This script doesn't seem to work for me. I installed the recommended extensions too, but for some reason YouTube's theater mode shows the player buttons too. (This issue could be caused by some extension/userscripts that I use, perhaps).

I will do a test later using only this userscript and the two recommended extensions. If the bug still persists I will provide better feedback.

By the way, what did you do to make your YouTube's ambient mode take up the entire screen?

Is the progress bar staying visible?

As I mentioned in the description, the progress bar may not be consistent across viewports. Hopefully someone can find a way to make it consistent, but for now, you may adjust the "bottom: -4.4vh;" CSS per your viewport dimensions. This allows you to move the progress bar up and down.

Ambient Light for YouTube is a customizable extension that is far prettier than YouTube's native ambient mode:
https://addons.mozilla.org/en-US/firefox/addon/ambient-light-for-youtube/
https://chromewebstore.google.com/detail/ambient-light-for-youtube/paponcgjfojgemddooebbgniglhkajkj

So... I just ran a test on a fresh and newly installed Chrome with only Tampermonkey (with this script) and the Ambient Light extension instaled. This time. the script behaved unexpectedly.

In addition to the progress bar being half inside the player and half outside, now when I'm not in theater mode it is partially cut off (as shown in the attached screenshots). It also don't collapse nor expand when I hover on the player.

As for the Ambient Light extension, I really appreciate it. I was looking for a way to enhance the look of my YouTube without changing the base design too much, and this extension fit the bill perfectly. Thank you ;)