XAFORUM-158: Improve display on mobile

* I didn't add 'margin: 0 65px;'. I just deleted 'min-width: 460px;', thus going on the previous line and had to delete some spaces in order to make the commit.
It's working for me. I used 630px, because that was the explicitly set min-width before, for a topic. And from that point decreasing, it was no longer responsive. Maybe I should put the width from tablets as a max-device-width (768px).