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 
@MorrisJobke if you’re ok with removing the border completely as well, I would be in favor of that. What do you think?