get out of high contrast mode for display

I just switched to a high contrast display to make reading screen easier pressing together the left shift left alt and prtsc keys. One of the forum threads says this sequence is a kind of toggle so that doing it again turns off high contrast. Didn't for me. Instead, it produced a menu asking if I wanted to turn on high contrast with yes and no options. I tried both and in each case the menu closed with no change.
In short, how do I get out of the high contrast which I find almost impossible to read (I'm 74 and have less than the best eyesight)?

Thanks so much for the solution