Oki idk howto do it on computer but there is a setting on ios where you can turn your screen to invert colour (in settings) ye thats basically all.. If you use something elsethen google howto turn on invert colours or something idk XD
Go to "type here to search" box at the bottom
Type in "Magnifier"
Zoom out one if it's too much zoomed it by pressing this symbol -
Go to settings and scroll down and check the box that says "invert colors"