Make filter button more visually apparent

[HaSistrunk]

Is your feature request related to a problem? Please describe.

Usability testing indicated that users did not immediately see the filter button on the search results page.

Describe the solution you'd like

It may be enough just to darken the border color to $deep-navy to match the text color (as is common in our other button styles).

Describe alternatives you've considered

We could also change the button color.

Additional context

I wouldn't advocate we do anything too dramatic since this hasn't been an issue we noticed in past testing.