Press ALT, SHIFT, and PrtScr at the same time. Select Yes to the popup that opens.
The Alt and Shift will only work if you press the Left one.
Or just go to Control Panel/Personalisation look panias said.
If you don't have the High Contrast Themes, download the Attachment I included and extract it into a Empty Folder on your Desktop. Then head over to C:\Windows\Resources\Themes\Ease of Access Themes and place it in there.
If you do not have Ease of Access Themes inside, you can create it.