greyed out screen 5 month old imac

and yes i having been jerking around with the settings in safari. comes on OK when new window is opened, one second later greys out?---Firefox seems to be ok__it is something i have done, but what?---made some changes as to david a. coxs Yosemite tutorial however i have mavericks so therein is the problem. but what is the solution.? thank you