Gammy, an adaptive screen brightness GUI tool that that was originally only available for Microsoft Windows, was ported to Linux (X11 only) recently.
The Qt5 application takes a screenshot periodically, then gradually adjusts the pixel brightness based on the screen (screenshot) contents, dimming the screen if its content is too bright, or brightening the screen if its content is too dark. This is especially useful for reducing eye strain when switching between dark and light windows.