Code snippets design tweaks

[simon-curtis]

I am putting forward a suggestion to the code blocks, making them rounded and have a softer drop shadow I feel fits the branding more, with the rounded points on Lucy and the general soft colour pallet.

Note: the colour change for the code snippet brackets icon must be from another commit

[lpil]

Thank you.

This looks like it's using nonstandard colours and is changing the code styling again, so we need to decide what to do alongside these two issues:

• Fix deviations from Gleam branding #70
• Improve contrast of the code on dark theme #59

Whatever we do here we'll need to carry across to the other websites.

[simon-curtis]

I have changed it back to --black for now. Meaning the only change is the rounded corners. I will request that the --deep-sea-blue be added to the branding as I think it's a really nice intermediary colour that sits nicely with the colour scheme