Hi,
There is no shortcode to allow toggling the dark mode using a custom button. The plugin comes with a sticky toggle button that you can configure to some extent.
For the sticky button, you may use negative values for positions like -15 to make some part of the button off-screen. Also, you can control its width, height, font size and border radius. You can find these options in “Appearance” > “Dark Mode”.
I might release the custom button feature to toggle the dark mode in future updates.
Thanks!