Our default theme is based on the “Light Scheme” colour scheme. It seems to be working, but:
While viewing admin > customise > theme, I can see “Light Scheme” in the drop down list.
However, there’s no “Light Scheme” listed under admin > customise > colors.
Is this because the Light Scheme is the system default, so it’s not listed on the colors page?
I’m unsure if this is normal.
Yes, I believe that’s how it is intended to work. If you want to get a copy of the Light Scheme to modify, you can follow Customize --> Colors --> + New then pick Light Scheme. It should be modifiable on Colors and selectable in your themes after that.
Thank you. I was a little concerned that the Light Scheme had been deleted or renamed and that this might affect users at some point. Sounds like that’s not the case 
I had exactly the same question - and assumed I was missing some files! In theory then the light scheme is the default and you only need to create it if you wish to make custom changes.