You can change your Space’s color or theme by clicking the “edit” button that appears next to your Space name when you hover in your Sidebar and then selecting “theme” from the drop down menu that appears. You can also make changes by simply right-clicking on the empty space in your Sidebar and selecting "theme" from the drop down menu. Either way, the Theme Picker will appear and you can change the look and feel of your Space.

You can also change Arc's theme to Light, Dark, or Automatic (Arc’s theme will match your device’s theme) from the same Theme Picker. Simply select either the stars (automatic), sun (light), or moon (dark) at the top. Just remember - this applies to all of your Spaces, not just the one you’re currently in.