[mod] simple theme: adjust margins

* index page: margin top is 24% of the view port.
  avoid to scroll a small screen,
  center the content in the middle of the screen

* link to preferences at the same height same the input fields

* increase the category tab heighs

* increase the margin bottom of the query field

* in the results, change the h3 margin top and bottom to 0.4rem (6px)

* move the back to top button slightly on the right when the results are only images