Currently, the color scheme of the desktop and the mobile platform cannot be changed. Alternative color schemes for the desktop and mobile platforms are in the works. However, the color scheme of the web browser platform can be changed.
Web Browser Platform
The web browser platform has two color schemes: Dark and Light. To change your color scheme, go to the Settings menu (click More>Settings). Once in the Settings menu, select Additional Preferences, then choose a Color Scheme in the drop-down menu.