I did see that post but I wasn't a huge fan of just inverting the styling. You're correct, the white square is not visible when doing the CSS invert.
The screens on this post we're built using the App theme config, which is when the white square comes in.
I will just wait for the below ticket to be completed for light/dark mode switching. Being able to switch between themes via user input is the ideal solution.
Expose Themes inside the Retool Environment