Even if you're using dark mode your eyes can still hurt by bright images/videos. This addon places a dark overlay infront of everything on the page to dim the brightness.

Toggle Dim (All tabs): Ctrl + Alt + N (Linux/Windows), ⌘ + ⌥ + N (Mac)
Toggle Dim (Active tab only): Ctrl + Alt + B (Linux/Windows), ⌘ + ⌥ + B (Mac)

You can change the shortcuts from about:addons -> Manage Your Extension -> Manage Extension Shortcuts. Settings
All tabs When this option is selected, the states will be shared between the tabs that is set to 'All tabs'. If you change the state (dim/undim or change the dim level) of one of the tabs, the change will be applied to all tabs that are set to use 'All tabs' option.
This tab only When this option is selected, the tab will have its own state and it will not be affected by other tabs' state changes.
Changing the settings
From 'This tab only' to 'All tabs' The current tab's state will be applied to all of the other tabs that is set to 'All tabs'.
From 'All tabs' to 'This tab only' The current tab state will be preserved, but further state changes will not affect the other tabs.
Default settings

All tabs New tabs will share the same state with the other tabs that are set to 'All tabs'.

This tab only New tabs will have their own state and it will not be shared with the other tabs.
Changing the default settings Changing the default settings have no effect on the existing tabs. It only effects the new tabs.