SkinConfiguration
IHasComboColours
`IHasComboColours` was already mutable (via a strange `AddComboColours()` method) and exposing a straight list is easier to work with. `IHasCustomColours` is also similarly externally mutable (in a way which is not easily removable).
I don't get why this wasn't resolved in the first place when this file was originally written. *sigh*
Also exposes functions to modify the internal list (AddComboColours, ClearComboColours)