Switching to light/dark modes

[runnermann]

Modern and popular UI's include light and dark modes. In a survey of competitive software, this seems to be a MUST have.

We've implemented light and dark modes in the rest of our software, but our forms lack this ability. I have used CSS to make changes. But these must be compiled with the library. This would not allow an end user to swap back and forth between modes.

Any suggestions on where to begin to implement this change would be appreciated. I upload my versions of this project to my github account. @runnermann