Sorry if this is obvious but I cannot locate the relevant section in the documentation...
How can I set colours of widgets in RGB hexadecimal formats?
These do not seem to work:
rgb(#aabbcc)
rgba(#aabbccdd)
rgb(#aa,#bb,#cc)
rgba(#aa,#bb,#cc,#dd)
The editor used decimal rgba(1,2,3,4)
but my brain does not work that way