A lightweight JSON file containing:

Here is the most common point of confusion. If you search expecting a .crx file or a theme store button, you will be disappointed.

While there isn't a single button to "download a theme for all tabs," the Arc community shares "Boosts" via link. You can click a shared link, and the Boost is instantly applied to that site.

Use a color picker tool (like Lightshot or Sip) to grab colors from your desktop wallpaper. This creates a unified desktop aesthetic.