Your comments
I have a Windows laptop. F11 and F12 darken or brighten the screen.
I should also note that pressing and holding does not clear that screen - it just pops right back up again and again and again. This makes your site unusable. Also the word "cookies" was dropped from the above post.
Customer support service by UserEcho
This appears to have cleared up for me - have not seen it for at least 2 weeks.