Update template.css

Change position of navbar to static rather than fixed, permitting scrolling of navbar and connected dropdown menus, on desktop view (which conforms desktop with tablet and phone views).
Make optional change in phone and tablet view that conforms with stick toolbar of desktop view.
This rule comes from Bootstrap. See https://github.com/joomla/joomla-cms/blob/staging/media/jui/less/navbar.less#L174.
Since you aren't allowed to change the Bootstrap files, you will have to add your suggested rule to https://github.com/joomla/joomla-cms/blob/staging/administrator/templates/isis/less/template.less.