With the new composer rolled into tests-passed I was just looking at colour schemes and found that selecting the light theme exhibits some strange behaviour:
Customize > Themes > New
By default the scheme appears as:
If I select the dark theme it appears as:
And if I select the light scheme again:
Prior to rebuilding I was seeing a similar problem with Customize > Colors, where I could create new schemes based on dark, but if I tried to clone the light scheme the +New button was active, but did nothing.