GTK3: CSS: use same font sizes as GTK2 did

If you want to customize this (on a per-user basis),
create file named ~/.config/darktable/darktable.css
with following content:

@import url("/usr/local/share/darktable/darktable.css");

* {
  font: Sans 10;
}
... <rest omitted> ...

That will bring font size to what it was before this commit.
GTK3: fix viewswitcher styles