Small adjustment for sidebar toggle on mobile

[thupsi]

#14910 introduced some nice improvements, but IMO the sidebar toggle has become a little too inconspicuous, and is looking a little "cramped". I'm suggesting that:

• The spacing above and below should be adjusted a little.
• Optionally it could display as a button, like before, in order to be a little more prominent, and in that case it could switch to full-width below a certain breakpoint.

It goes without saying that it's just a small detail, so no hard feelings if you don't agree 🙂