Yeah @Jagster, I think you have the right idea.
@ArunROG87Razer Discourse has two separate but related features that are relevant here.
Themes are modified versions of the user interface that can change not only colors, but also layout and functionality. Creating a new theme can take anywhere from hours to months depending on how complicated it is. Installing an existing theme (such as those listed here), takes a couple minutes if you have a URL to the theme’s Github page.
Color Schemes are alternate sets of colors. They can either be defined by themes or created separately by in the admin interface. Creating a new color scheme takes only a few minutes (plus the time it takes to decide on the colors).
Generally a “dark mode” only involves a color scheme. One caveat is that Discourse creates larger color palettes from the handful of colors defined in the color scheme, and then uses the generated palette to color the whole site. For instance, in this screenshot you can see how different shades of the primary color have been generated.
A theme allows for more control over what colors are used where, and also lets you include other colors outside the auto-generated palette.