Remove background from subhead on large screens

[chmst]

When we started, we had not a horizonal filter bar and tabs were at the bottom of the screen. So we used a background and border to make a visible box for the toolbar.

Now this box is superfluous on large screens.

Summary of Changes

Remove the background and border from subheader but only on large screens.

[brianteeman]

LGTM

[bembelimen]

I like it, not sure if we need the background on mobile, perhaps we can remove it there, too?

[chmst]

It is a matter of taste. But let's try "less is more" :)