Currently the passepartout color is hardcoded to black. While a sensible default for cinema, it may make less sense for other media, whether video, print, web, etc. It greatly affects viewing conditions of the image and should be user-selectable, much like painting programs allow.

This patch exposes the passepartout color as a theme color.

Before:

After, with color set to gray:

Suggested by scopelma on Right-Click Select.
https://blender.community/c/rightclickselect/4z41/