Themes app not showing preview for installed GTK themes (Window borders and Controls) in Cinnamon

Late answer, but I hope it could still be useful. Most probably that happens because no theme's thumbnail is provided for cinnamon. See /usr/share/cinnamon/thumbnails/gtk-3.0/README:

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?
---------------------------------------------------------

- 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/

and /usr/share/cinnamon/thumbnails/metacity-1/README:

HOW TO MAKE A METACITY THUMBNAIL?
---------------------------------

- Install ImageMagick ("apt install imagemagick")
- Open a window (Nemo for instance)
- 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-east -extent 100x32 thumbnail.png

WHICH THUMBNAIL IS USED BY CINNAMON TO REPRESENT A THEME?
---------------------------------------------------------

- Cinnamon first tries to find a metacity-1/thumbnail.png file in the theme directory
- If it doesn't find any, it looks in /usr/share/cinnamon/thumbnails/metacity-1/