feat(highlight): use theme text colors

[TomJGooding]

The new highlight module for syntax highlighting currently uses the 'base' theme colors. Perhaps this was intentional, but to my eye this makes the colors in Markdown code blocks so prominent that they feel a bit out of place.

This updates the highlight theme to instead use the dedicated theme text colors, which I think looks more consistent.

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[willmcgugan]

I'd forgotten about those. Yeah, that looks a little kinder on the eyes.

[TomJGooding]

Here's a side-by-side comparison of the Textual demo to demonstrate:

Before	After

[TomJGooding]

I'd forgotten about those. Yeah, that looks a little kinder on the eyes.

I'm glad you agree as honestly I was a bit hesitant to make this PR. Probably I'm just a bit fixated on theme colors after working on my theme editor!

I'm not really a fan of the new inline code colors either, but that's obviously just personal taste.

[willmcgugan]

How would you improve the inline code style?

[TomJGooding]

@willmcgugan What was wrong with the old colors? I'm used to inline code looking like this so the change to warning colors just looks strange to me.

[willmcgugan]

Previous version was bold and dim, IIRC. I didn't feel it had enough emphasis. And obviously it already has a monospace font, so there was nothing to say this was code.