Location: Appearance → Customize → Global → Colors → Color Skin
The Spacious theme allows you to select Skin color for your site, i.e., Light skin or Dark skin. Normally the site adopts a Light skin color but looking at the ongoing period, most people prefer to read websites in the dark skin mode.
For a better user experience, all the well-known companies have adopted dark mode, including Google, Facebook, and Instagram. Your website looks more professional and is better for your eyes in dark mode.