Related to #737.

#737 requests support for media queries to switch video files. That's great, and worthwhile, but I can also add an on-demand option to reduce brightness. This is not at all equivalent, but I could add something that essentially overlaid an opaque filter on the video and/or controls so that users could reduce brightness.

It has the advantage that it's a user-focused feature, but the downside that it's not seamless and would add another control to the interface. However, could be worth considering.