Hi @alessandromarani; welcome to the Dropbox Community and thanks for using Paper!
To turn off the dark mode on Paper, click on the question mark to the lower right corner of the window and then click on 'Dark Mode'.
From there, you'll get an option to set it to be 'On', 'Off' or 'Auto' (which turns on dark mode automatically whenever your device is in dark mode).
I hope this helps and please let me know if you have any more questions Alessandro!