This has two separate parts: - Branding color for the backend interface (changing system buttons and menus to company colors): V5 does not currently support this setting. It is planned for a future release. - Light/Dark theme: Yes, you can. Click the Preferences icon in the top-right system bar, then select your theme (Follow system, Light, or Dark) in Appearance. In the same area, you can also adjust font, font size, and interface density. Changes save automatically and follow your account—they remain the same on other computers. The brand color in Company Basic Settings > Catalog layout affects what customers see in the catalog and login page, but does not change backend interface colors.