How to turn off transparency in Windows 10

1. Open the Ease of Access settings by pressing the Windows logo key  + U.  Alternatively, select the Start Menu, then Settings, then Ease of Access.
2. In Ease of Access settings, select Display from the left column.
3. On the right, scroll down to the Simplify and personalise Windows section.
   
4. Select the toggle switch under Show transparency in Windows to turn off transparent effects.

Note: If this does not work it could be because your computer settings are managed by someone else (an IT department or administrator for example). If so, you will need to contact them to access these settings or for help.