Style the "skip to main content" link

[fuzzbomb]

The skip link is currently unstyled, and it causes a content jump when focusing it.

It has the core .visually-hidden.focusable classes, so the functionality is otherwise fine.

As a first cut, we could try copying the style from Bartik/Seven, and see if it still works here. That's a good style because it sits well next to the toolbar. Maybe change the colour to resemble one of the existing Umami buttons.

Later we could design a custom style if we like.

[markconroy]

deploying this now:

[fuzzbomb]

Great!