Hello,
fast-syntax-highlighting supports themes and highlighting of $(). This
opened a way to implement: use distinct theme for $() interior. Effect
is in attached file. Additional axample:

http://psprint.blinkenshell.org/cmdsubst3.png

The secondary theme is specified by simple entry in theme ini-file:

https://github.com/zdharma/fast-syntax-highlighting/blob/master/themes/clean.ini#L7

If (Z+x+) flag will be implemented in Zshell, it will be possible to
do all this very correctly, robustly.

Also check out 256-color themes, the difference to default "rigid" 8
colors is drastic:

http://psprint.blinkenshell.org/theme.png