Dark Theme - Trigger on toggle without needing page refresh

Requiring a page refresh isn't necessary, I discovered the other day. We could use messaging to trigger the javascript to make the theme appear.
https://developer.chrome.com/extensions/messaging

Start with the Chrome extension messaging documentation linked in the issue, then locate the theme toggle and the current page-refresh flow in the extension. The change is complete when toggling the setting causes the dark theme to appear immediately without refreshing the page.

Written by the indexing model from the issue text.