Focus state of the navigation panel items does not have enough contrast

This is not a pseudo class, we have a real class added. This background style is added at https://github.com/xwiki/xwiki-platform/blob/1d9d01326a83c9816d9a8cfe5697dc147b40f877/xwiki-platform-core/xwiki-platform-tree/xwiki-platform-tree-webjar/src/main/less/jstree/main.less#L7