In DMG Canvas's preferences, enter the Apple ID of your developer account. Because the underlying Apple tools used for notarization do not support two-factor authentication with the Apple ID, you will need to create an "app-specific password" for DMG Canvas to use with your account.
After that, in the disk image settings in the sidebar, turn on Notarize with Apple, and select your Apple ID. For the "primary bundle id" enter any value which describes the primary content in the disk image being notarized. This is used for reference by the underlying notarization tools.
During the build process, DMG Canvas will submit the dmg to Apple for notarization and wait until it finishes. Once finished, the dmg and any applications inside of it are then notarized, and the resulting dmg is then stapled with the notarization ticket.
Note that to notarize a disk image, the dmg must also be code signed.
Because disk images are display in Finder as normal Finder windows, the dark mode "support" they have is the same was what Finder has. When the window has no background image or color explicitly set (and is instead left at "None/Default" in the DMG Canvas settings), the background of the window in Finder will change between white in light mode, and dark gray in dark mode. The names of files will also change between black and white respectively. If a background image or color is specified though, the file names will always remain in black. Unfortunately there is no choice to have a different styles of disk images for the different system appearances.
By changing the preview appearance with the popup menu in the bottom left of the design view, you can view the approximation of how the disk image will look in either appearance, so you can ensure there are no unexpected issues with readability and get a sense of how your style choices work with a dark mode window frame.