A logo drawn in dark ink vanishes against a dark header. Upload a light version as Logo for dark mode in Admin → Settings → Branding.
Both logos are sent to the browser and CSS chooses between them. Convoro cannot choose on the server, because a visitor whose theme is set to Auto has their mode decided by their own operating system — something the server does not know when it builds the page.
Leave it empty and your main logo is used in both modes.