User-customizable letterboxing background
In #41917 (closed) we added a themed gradient to the letterboxing background to make it appear more intentional.
This caused some users to complain for the very same reason:
The gray letterboxing in the past was good as it wasn't noticeable, but now it sticks out like a sore thumb and displeases my eyes - as it's pure white with a light theme and light purple with dark theme. [...] I believe such regression would be fixed if there was a way to set the letterbox background colour to a wanted one.
Maybe we could make the background (color? gradient? image? all the 3?) customizable in the preference UI section we created in #41916 (closed).
No Fingerprinting implications here (the background is chrome-side and couldn't be detected from content), but Accessibility and UX.