At school, I accidentally turned on high contrast and am unable to turn it off. Control panel is disabled, personalize in context menu is disabled, batch files won't work...
If you were like me. What the trouble is, is not that the High Contrast system theme will not turn off; it is that it will not turn off in your Browser window.
Solution; (for Chrome, but the same principle should work for all browsers) Enter your browser settings drop down, top right of the screen. Select "more tools" - then find and turn off the High Contrast Extension. You may delete if you are of the mind.