Description of the new feature/enhancement

Pull request Add a setting to flash the pane when BEL is emitted #9270 implemented a visual BEL indictor on the working terminal pane. It's currently a white flash, which works well against a traditional dark-mode background. However, on a light appearance this basically only lightly flashes the text, making it a little hard to notice!
Is there a possibility this could be modified to work with higher contrast light-mode users? Thank you!

Proposed technical implementation details (optional)

I'd probably set (and store?) an isBackgroundLight bool somewhere iff the background color of the pane is "light". If it is, and the BEL handler has the window flag set, the pane would flash dark on a BEL instead of light.

A quick way to find isBackgroundLight may be to just see if the average of r, g and b is higher than 127. I can't imagine enough people have #888888-ish terminals for this to warrant being a manually-defined setting.