Looking at the code I have the impression that many UITheme methods should be moved to the their classes
Example:
UITheme>>newGroupboxIn: aThemedMorph label: aString
"Answer a groupbox with the given label."
^GroupboxMorph new
font: self labelFont;
cornerStyle: aThemedMorph preferredCornerStyle;
hResizing: #spaceFill;
vResizing: #spaceFill;
label: aString;
yourself
Why this is not a creation method on GroupBox?
Then the trait TeasilyThemed looks suspicious to me.
