I have the same issue! The Roku Home Screen is way too bright, its like the Backlight goes up to 100% when you hit the Home button on the Roku Remote. This happens to me on 2 different Roku TVs from 2 different manufacturers (55" TCL and 65" Element Electronics).
There is no way to change the Backlight level on the Roku Home Screen, it defaults to 100%. This is bad because it unnecessarily shortens the life of the Backlight LEDs (at 100% brightness) if someone leaves the Roku Home Screen and Roku Screen Saver ON most of the time.
The best way to experience this issue is to watch a movie in a dark room, and set "Normal" or "Dark brightness" in Picture Settings for that movie. During playback, hit the Home button on the remote and notice how much brighter the Roku Home Screen is compared to the movie/input. I think the solution for this issue is for the Roku Home Screen to inherit the Backlight setting from previous input or app/movie. Or set the Roku Home Screen backlight setting to anything less than 100% (maybe 70-75% would be good).
Roku, please fix this Roku Home Screen too bright issue!