Can I create a theme, instead of colour scheme?

blunt answer is, you guessed right: no.

as you know already you can only take and even customize the color theme; you can also choose the widget style, although I don't recommend that much, as most of these are troublesome, to say the least, specially if they're based on gtk+... stick with "Fusion" and all good will prevail ;)

you may also tweak the meter colors but that's kinda user preference... all else, like geometry, window menus and toolbars and dockables layout and visibility are application state options that gets saved and recalled on the next time the program is run, and some other things as well but unrelated to screen/visual display whatsoever...

maybe you ought to explain a little better what you mean with "workspace"? in my book, a "theme" is not (even remotely) related to a "workspace", rather only to a "visual scenery" ?

cheers