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 HOW TO MAKE A GTK THEME THUMBNAIL?
 ----------------------------------

 - Install ImageMagick ("apt install imagemagick")
 - Open a terminal and run "cinnamon-preview-gtk-theme mytheme" (where mytheme is the name of the Theme you want to thumbnail)
 - Press Alt+Print-Screen to take a screenshot of the window
 - Go to your Pictures directory and move the screenshot to ~/screenshot.png
 - Open a terminal and type:

	cd
	convert screenshot.png -gravity north-west -extent 120x35 thumbnail.png

 WHICH THUMBNAIL IS USED BY CINNAMON TO REPRESENT A THEME?
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 - Cinnamon first tries to find a gtk-3.0/thumbnail.png file in the theme directory
 - If it doesn't find any, it looks in /usr/share/cinnamon/thumbnails/gtk-3.0/