Smart has support for dark and light mode out of the box.
By default, Smart will adapt to your user's system preference. For example, if they prefer dark mode, then the site will automatically display in dark mode. If your user doesn't have a preference set, then the site will default to dark mode.
If you want your site to default to light mode instead, go to Settings > Design > Site > Dark/Light Mode and set your preference.
Your user can override the default behavior by choosing the opposite mode from the Overflow menu. This preference will be preserved on subsequent visits.