Changing the icon within a Chromium extension, depending upon the OS' dark mode status

changeIcon.js

// Functions to change the app icon depending upon light or dark mode

function updateIconLightMode() {
  chrome.browserAction.setIcon({
    path : {
      "32": "assets/appicons/32.png",
      "24": "assets/appicons/24.png",
      "16": "assets/appicons/16.png"
    }
  });    
  }
  
function updateIconDarkMode() {
  chrome.browserAction.setIcon({
    path : {
      "32": "assets/appicons/32-white.png",
      "24": "assets/appicons/24-white.png",
      "16": "assets/appicons/16-white.png"
    }
  });    
  }

// Icon changing

if (window.matchMedia &&
  window.matchMedia('(prefers-color-scheme: dark)').matches) {
    // SET DARK MODE ICON
    updateIconDarkMode();
} else {
// SET LIGHT MODE ICON
    updateIconLightMode();
}

1. Add the above to your extension folder
2. Within your extension (e.g. index.html) add <script src="assets/js/changeIcon.js"></script>

Of course, be sure the icon paths are valid. To see this in action, change your OS dark mode off and on and click upon the extension icon. (Within macOS, go to System Preferences > General and change the Appearance).