Marcel Taeumel uploaded a new version of System to project The Trunk:
http://source.squeak.org/trunk/System-mt.1142.mcz

==================== Summary ====================

Name: System-mt.1142
Author: mt
Time: 5 March 2020, 1:17:57.313611 pm
UUID: c0bcbaf5-9833-b641-ae8a-8e03d89bb84c
Ancestors: System-mt.1141

Adds support for setting a value in the current UI theme similar to retrieving it like this:

self userInterfaceTheme myBackgroundColor: Color black.

=============== Diff against System-mt.1141 ===============

Item was changed:
  ----- Method: UserInterfaceThemeRequest>>doesNotUnderstand: (in category 'lookup') -----
  doesNotUnderstand: aMessage 
  	"Look up the visual attribute specified by aMessage's #selector in the current theme for the current target object."
  
  	aMessage numArgs = 0 ifTrue: [
  		^ (self theme get: self target class -> aMessage selector)
  			ifNil: [(self theme respondsTo: aMessage selector)
  				ifTrue: [self theme perform: aMessage selector]
  				ifFalse: [nil "unset property"]]].
  	
+ 	aMessage numArgs = 1 ifTrue: [
+ 		^ self theme
+ 			set: self target class -> aMessage selector asSimpleGetter
+ 			to: aMessage arguments first].
+ 		
  	^ self theme
  		perform: aMessage selector
  		withArguments: aMessage arguments.!