i just tested twentyfifteen and the quick tag bar shows up fine as long as you have post formatting in enabled in settings>forums.
i also tested the tinymce tab plugin and it seems to work well together.
does clicking any of the tabs not show the toolbar , sometimes in the past it would show nothing until you click the tabs.
and also make sure to double check just in case you cant see them.
the buttons and button font color could be white based on the image you posted.