Alternatively, instead of assigning the region face to it as a defvar, you could use the same color values as region, but making semantic-idle-symbol-highlight-face a defface.
> If someone recommends a good light (white) background and dark (black)
> background color to use, I can apply them.
I think subdued is best, so a possibility is:
For dark background:
:background "#002220" :foreground "white"
For light background:
:background "#ddfffd" :foreground "black"