pert:
The folder structure should look like this:{sketchbook folder}
|_theme
|_buttons.png, etc.
|_theme.txt
|_syntax
|_default.xml
My sketch folder does not have such a folder within. Are you saying I have to put it there manually?
We *are *talking about the folder with the sketches I've created, the libraries and such, correct?
pert:
Are any of your modified theme settings having an effect?
Some. Here's a snipped view. The blue and green and orange/red carried over from the previous dark theme - which I did not alter. The parens after setup and the baud rate for serial.begin are there, even if they can't be seen - it's a black on dark gray thing.
I did find in my search an option for a built-in dark theme 
but I could make it work.
Edit: I was able to relocate the built-in dark theme - craftily hidden right at the top of the program files (x86)\Arduino\lib\theme\syntax folder - and activate it. Those devils!
There are two files in there, 'default' and 'dark'. I just renamed them to 'default orig' and 'default' respectively.