Aqua is really not considered a serious or efficient user interface if you talk to most people who want to get work done without thinking that they are using a computer. Many PC users I talk to like its looks, but dont like the way it slows you down.
For example, if I type a new document in TextEdit and hit Cmd-S, type a file name, and Return to save it, i am done typing all that before the sheet has finished sliding down. Now, I have to wait for it go finish going down, access the HD for directory info to display, and slide back up.
On Mac OS 9 or Windows the animation steps don't exist. On OS X is it pretty? Yes. Productive? No. It's reasons like these people I know are reluctant to upgrade, and those who have (like me) really want the option to disable these kinds of slowdowns.
Back on topic: Themes don't change sheets - and that shows the real problem. Apple holds the interface reins tight in hand, and Aqua is notoriously hard to hack into. Themes have only very limited control over the interface, and the OS 9 days of 3rd party interface hacking are gone - Unsanity is pretty much the only company knowledgeable enough to do anything to the OS X interface. Apple limits interface change like never before sadly, and the options one has are really small compared to XP or OS 9.
I'll give Apple one bonus point though: At least the slow-rendering drop shadows are at all configurable (through 3rd parties only though). I only wish Aqua didn't rely on them for window borders. Try ShadowKiller and you'll see what I mean - the windows all become white where they overlap - the slow drop shadow is required to distinguish between them. Now run Let1KWindowsBloom - even in Classic, OS 9 is MUCH faster at spawning windows because of the drop shadow.