Minor styling update to prove a few theme things

[jonathonherbert]

What does this change?

Makes a few more themeing decisions to bring us closer to a Composer-ish style.

These are mainly to prove that we can do a few things –

• theme both sass/emotion-based components with EUi variables, as EUI currently uses a mix (we're stuck overriding in both sass and Emotion contexts for now, but EUI does have a clear roadmap that's seen significant progress in the last few months – search that page for css-in-js conversions)
• replace icons

It also fixes an issue we were having where Emotion-based globals were overriding sass globals.

Before	After

How to test

Take a look. A bit closer to the angular, austere beauty of our other tooling?

[jonathonherbert]

@phillipbarron yes! Although, I actually can't get package-lock to change when I run npm locally? 🤔

Edit: solved by pinning the node version.