Update focus styles to match GOV.UK Frontend 3.0.0

[NickColley]

What

The current focus style and colours are out of date when compared with GOV.UK Frontend 3.0.0, which means they do not meet the new WCAG 2.1 non-text requirements.

Further detail

We should make sure the styles match the guidance on focus state: https://design-system.service.gov.uk/get-started/focus-states/

There is a related discussion here: #329 but we may decide to do something more pragmatic in the short term.

Done when

• Focus styles match GOV.UK Frontend inputs