As a hobby developer, when testing dark theme vs light theme under MacOS X, I have those moments where I have to switch back and forth between both themes quite often.
Going into the System Preferences to toggle theme, choosing the Appearance option, and then selecting the desired theme, does indeed work, but I wanted just a “one click” theme toggle.
To help myself with that, I created a small application that does exactly this: it toggles to “the other” theme, with just a simple click.
So when Dark Theme is active, it switches to the Light (Aqua) Theme. If however the Light Theme is active, it switches to Dark Theme.
The application is super simple, and has no interface. Just start the App and it changes theme and then closes itself.
Normally, this App would just sit in your Applications directory, with the optional shortcut in your Dock or even on the Touch Bar (if your Mac has one).