Design controls polish

This fixes a visual issue with the controls bar. Before, it looked like the controls bar was part of the sidebar because it had this strange grey color.

Now it’s clearly part of the content as it’s white. I also reduced the transparency a bit for less overlay conflicts cc @PVince81 

This also reduces the grey-in-grey design a bit cc @jbtbnl ;)

Please review @owncloud/designers 
Currently it looks like crap. We should remove the border or make it a bit darker. With this color it looks like a half of all other border on the page and misplaced (see image above)