XWIKI-11085: Flamingo Color Theme not present in subwikis, after an upgrade from 6.1.

 * Change the code of the displayer to not rely on the XWikiPreferences 'colorTheme' property

(cherry picked from commit f6c3cd3363e682a7a3f83f4b17ccd72cd99a4fe7)

Conflicts:
	xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-ui/src/main/resources/XWiki/ColorThemePropertyDisplayer.xml