Hello!
Where do I change colours for tree panel frame & scrollbars (see attached image)?
They use the system colours. You can change them via the Display control panel.
Any known reason for leaving them off of the DO configuration?
Just my guess:
I think the assumption is that people won't want to change those things from the standard colours. Most of the things you can change in Opus don't have system-defined colours/backgrounds (or didn't when the first PC version of Opus was written, at least) or are things like folder-specific background images where it can be useful to have a watermark for particular folders or types of folders (which again isn't definable via Windows).
Similarly, you can't change all the colours used to create the Rename, Preferences, etc. dialogs since it's assumed you want them to look like a standard dialog and if you don't like the colours then you'd want to change them system-wide for all programs and/or use a different Windows visual style ("theme").