Some "Theme" / "Monochrome" components in code examples aren't very visible

[GCHQ-Developer-847]

Summary of the bug

We've given components withtheme="dark" in "Theme" / "Monochrome" code examples a dark background to make them visible in light mode. However, the theme="light" examples don't have a light background when dark mode is turned on on the website, so they are quite difficult to see / there is not enough contrast.

🪜 How to reproduce

1. Go to the "Theme" or "Monochrome" examples on the Code page for a component e.g. link, divider.
2. See that the examples with theme="dark" are quite difficult to see

📸 Screenshots or code

🧐 Expected behaviour

I think it would be best if the components with theme="light" have a light background when in dark mode, just like those with theme="dark" have a dark background when in light mode.

Additional info

There are probably a few component pages for which this is an issue so it might involve looking through all of them to check. However, it's only an issue when we are showing two variants in one example i.e. when the dark background in light mode is only behind one of them, e.g.:

[GCHQ-Developer-847]

Noticed that the stepper Theme example doesn't have a dark background - would be good if this could be sorted as part of this ticket

[GCHQ-Developer-847]

Sort out the section container examples as part of this (#1275).

This ticket should be to just do a general check over all code examples in dark mode to make sure that they look ok. Fix any small issues, raise a ticket for anything big / ic-ui-kit changes.