Header: Add background to logo on hover

[ryelle]

Fixes #366 — Add the hover color behind the logo.

Default	Blue	White	Classic

[jasmussen]

Thanks for doing this. It's a bit hard for me to compare the before and after, here's my best attempt:

From the inspector, this seems like it should be fine, because the padding of the logo button is uniform:

So it looks good (ship it).

When I initially created 366, though, I thought I recalled a non-uniform padding that might have needed finessing. But that may have been from the figma mockups: