Fix .scss files to work with Sass 3.4.5.

Two bugs here, relative to Sass 3.4.5 (Selective Steve).
The first bug, reported at
https://github.com/dsociative/2048/commit/80258ad81716fd0dc263a3b0e154c7624afe1a0e
is that Sass apparently requires interpolation braces #{} around
a mixin parameter used on the left side of a colon.

The second bug, visible at that same commit, is that when you
refer to an unbound variable inside a mixin, Sass apparently
now looks them up in the global scope rather than in the local
scope of the "caller" of the mixin. For example,

    @mixin foo() { bar: $bar; }

    $bar: 1;
    a {
      $bar: 2;
      @include foo();
      baz: $bar;
    }

gives `a { bar: 1; baz: 2; }`.

This may in fact be a bug in Sass 3.4.5. Upstreamed as
https://github.com/sass/sass/issues/1474