Resposta tardia, mas espero que ainda possa ser útil. O mais provável é que isso aconteça porque não há miniaturas de tema para a canela. Veja /usr/share/cinnamon/thumbnails/gtk-3.0/README :
HOW TO MAKE A GTK THEME THUMBNAIL?
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- 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/
e /usr/share/cinnamon/thumbnails/metacity-1/README :
HOW TO MAKE A METACITY THUMBNAIL?
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- 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?
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- 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/