Change blue background on nav to underline

[EilidhIsla]

Currently the blue background and focus states cannot exist simultaneously. Copy the gov and NHS pattern on a line indicating the current page.

Also remove the padding by default at the top of heroes.

[bartfeenstra]

With the focus states being a background color and an underline, and the proposed active link styling also having an underline, how would the active and focus styles coexist?

[EilidhIsla]

One is at the bottom of the box, one under the text. See GOV for example