Fix deviations from Gleam branding

[lpil]

This PR introduced some extra colours. We need to bring it back into being consistent with the core branding so that it does not define any new colours.

We could promote some colours into core if useful, but we should look to remove extra colours generally as new ones keep getting added.

[simon-curtis]

As per the mentioned in #73 I'd like to suggest --deep-sea-blue: #202431; which is a variant on the current --underwater-blue to replace the --black colour as the dim colour due to how soft the colour pallet is

[lpil]

Sorry, what is the screenshot there showing?

[simon-curtis]

The difference between the current --black colour and my suggested --deep-sea-blue

[lpil]

To be honest I'm finding it hard to know what's best here! Visual design is not my forte 😓