Changing the Fullscreen Mode Background Color 

-Bring up the CSS page:

Alt + Space or F12

 
-Add the following CSS comand:

#slideshowOverlay { background-color: #FFFFFF; }

 - Customise and Save.
 
NOTE: The switch in fullscreen mode switches between BLACK and WHITE by default.
Design your Page > Advanced Mode > Background Color in Fullscreen Mode

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