ostrich-theme.el: gold cursor

* Makes cursor background gold rather then yellow.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
ostrich-theme.el: gentler colours for selections

* Sets gentler greyish colours for `primary-selection`,
  `secondary-selections`, and `region`.  Achieves nicer synergy with
  highlighted current line (`global-hl-line-mode`) also in greyish
  colours, improving UX when working with `transient-mark-mode` and with
  `helm`.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>