Update code snippet colours to be more accessible

### Description

@jen-machin highlighted some issues with the code snippet colours, we need to do more testing on them and then update accordingly.


Suggestion that the code snippets might work better with a different background.

### Why this is important?

They should be accessible to everyone, it's a minimum standard for us to meet

No file or test is named. First locate the code-snippet styling in the Quarto slide template, then test the current colours against accessibility contrast requirements and identify a suitable background; done means the snippets meet the project’s accessibility standard.

Written by the indexing model from the issue text.