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.
The Gammy settings allow setting a minimum and maximum brightness, and an offset (the offset adds to the screen brightness, with a higher value meaning a brighter image). Also, because it conflicts with Redshift, the Gammy developer decided to add basic temperature control, which you'll also find in the application settings.