In SciTE 3.3.6 command substitution will be simply drawn as block/string as seen on the screenshot in the attachments. Maybe it would be possible to use syntax highlighting there too.
Sure, it's one of the remaining missing bits for the shell lexer. If anyone wants to do this, please feel free, this can be discussed on the list too. We can at least keep this feature ticket for now.
It is necessarily a very low priority item for me because strings in actual bash scripts are mostly pretty uncomplicated.