The color theme custom displayer is not used in the administration by default

When oldcore creates the XWikiPreferences class, no custom displayer is configured.

However, the Administration Application provides one.

XE comes with a XAR that contains an XWikiPreferences document with the displayer used, but this XAR might not be installed (example: during integration tests, or possibly new flavors).

In my opinion, since it's the Administration Application which provides a custom displayer, it should be that application which configures the class to use the displayer.