Split styles into multiple CSS files.

[groundedsage]

Create a base CSS file which is shared across pages such as layout, typography and common above the fold rules.

Then the rest of the rules into a singular file. Eventually will review splitting this further.

[groundedsage]

This is less important now with atomic styles. Will review this much later to see if it provides any value.

[groundedsage]

This has evolved into a different issue with the introduction of atomic styles.