[Sort] Array is ugly with opacity

[andrewseguin]

The sort header's color is set as an RGBA value: rgba(0,0,0,.54)

This causes the arrow's parts to have a transparency applied. Would be better in this case to have a color applied with an opaque value.

https://stackblitz.com/edit/angular-ac6kex?file=app/table-sorting-example.css