[css-color-adjust-1] `ForcedColorsModeAutomationTheme` should not be a Web IDL enum

[tidoust]

The spec defines a ForcedColorsModeAutomationTheme Web IDL enumeration to create a possible set of values for internal algorithms. That seems like a mis-use of Web IDL: a Web IDL enum declares a type, for use in other Web IDL constructs. Or is there any plan to re-use or expose that enumeration somehow through a Web API?