Cortex SASS variable auditing

[ElliottAYoung]

As we approach the end of V2 and and dawn of V3 it's very important that we get our ducks in a row in terms of our style variables / our adherence to the style base that Susan drew up for us. Basically we need to go through our SASS to make sure that every variable that's there should be there and ensure that we are using our variables everywhere we can.

Hopefully it'll make our lives easier going into the Cortex React Renaissance!

[toastercup]

I think these variables are pretty well set for now - we'll be overhauling them entirely for the transition to MDC-Web