HTML code snippet/block highlighting

[sra405]

Primarily fixing dark mode colours for use in the code snippets and blocks in instructions.

The editor input panel will be in light mode for the integration initially so if it's quick and easy to apply there too for the standalone its worth doing but if not raise another ticket to apply the same styles at a later date.

[sra405]

have a play with the python darkmode syntax highlighting to see if we can utilise the same colours/theme

[create-issue-branch]

Branch issues/815-HTML_code_snippet/block_highlighting created!