Propose a dark theme

https://github.com/xwiki/xwiki-platform/blob/7b3680f01da34b632c04524668cfda2f35a43360/xwiki-platform-core/xwiki-platform-skin/xwiki-platform-skin-skinx/src/main/java/com/xpn/xwiki/plugin/skinx/CssSkinExtensionPlugin.java#L87

-> the code that creates the link tags for skin extensions. We could duplicate it to give both color themes.

https://github.com/xwiki/xwiki-platform/blob/3746464546e01f2f1fd468f922e754c01800e44a/xwiki-platform-core/xwiki-platform-ckeditor/xwiki-platform-ckeditor-ui/src/main/resources/CKEditor/InlineEditor.xml#L141-L148 Sometimes css will be loaded directly from javascript. We need a solution so that the CSS loaded like this uses the correct color theme.

Nikita Petrenko 

It would mean that each user individually has the choice between a light and a dark theme.

The admin still controls what are those themes but this means that the interface could be radically different between two users.

Note that the admin can already set a dark theme on its wiki. We don't have a good support for any of them yet though (at least not as good as Iceberg).