SHELL

linux mint 20 dark mode

1 - click on LM icon (the left bottom corner)
2 - into the field type: welcome (fr: Ecran d'accueil)
3 - select "Welcome Screen" (fr: Ecran d'accueil)
4 - in the new window, select "First steps" (fr: Premiers pas)
5 - in "Desktop Colors", click on the moon (fr: cliquer sur la Lune)
That's it ! (fr: Voilà !)