Once logged into the Live UI, in a browser, a menu is accessible. This can be found in two places depending on your screen size:
This enables navigation around the site as well as access to change the color scheme, enter Settings and log out.
By default the site will use the System color scheme, matching the users browser settings.
The color scheme chooser allows a Light or Dark color scheme to be selected.

The color scheme chooser is available to all users of the site.
This allows each user to change the site between Light and Dark color scheme depending on preference.
The colors used for the Light and Dark themes are set (by a site Admin) in Appearance Settings.
The background color of the window a user form is displayed in is set in the Run Appearance Setting.
Please Note
By default the background color for the Dark scheme is set to #fff (white).
This allows user forms, that have not been designed for a dark color scheme to be displayed.
To fully adopt the ability to display a user form in Dark mode in a browser, see Emulate Color Scheme.