First quick thought: What about renaming #userInterfaceTheme to #scopedInTheme or something similar that does not look like an accessor?
It is certainly better.
Now, since #userInterfaceTheme must always be followed by another selector (else bad things may happen), it seems to me that it should not be a unary method, but a binary one.
So what about
Object>>themed: aSelector ^ self userInterfaceTheme perform: aSelector
... which of course could be simplified so what we get rid of #userInterfaceTheme altogether (and actually of the full scoping mechanism):
Object>>themed: aSelector ^ UserInterfaceTheme current get: self class -> aSelector
The usage would be
BalloonMorph new themed: #color
where we now have
BalloonMorph new userInterfaceTheme color
I understand the latest looks better, but I do not think it is worth all the DNU shenanigans it requires the end user to be aware of...
Best,
Stef