refactor: Improve code highlighting

[palant]

This has two goals:

• Do not use JavaScript for code highlighting. Not only will it be problematic when JavaScript is disabled, it produces awkward visual effects when loading a page in dark mode (initial code highlighting is meant light theme).
• Make dark and light themes consistent. Switching between themes will currently produce wildly inconsistent highlighting - not merely colors change but also bold and italic text, also which parts of the code have different colors to stand out.

My initial approach was putting highlighting styles directly into the light-theme/dark-theme mixins. This unfortunately increases the CSS file size by 10kb because both of these mixins are used twice.

So instead I added the basic styles to _highlight.css and merely define the ten color variables in the mixins. This way the CSS file size even decreases slightly.

The downside is that changing/overriding styles is no longer merely a matter of running hugo gen chromastyles. On the other hand, people can now override specific variables fairly easily.

The other downside is that not all styles are usable in this way. I used the dracula style as a template, but simply taking color values from rainbow_dash for the light mode didn't work - this style uses color values that are very close to each other, relying heavily on bold text as distinctive factor. I decided to use manni style instead which is similar but has sufficiently distinctive colors.

Here is what light/dark mode highlighting used to look like in comparison:

Here is what it looks like with this change (only light mode changed):

[palant]

I added one more commit here which isn't really related, merely to avoid a merge conflict later. I noticed a bug introduced by #169 - the brightness reduction in dark mode will only work if the user explicitly selected the them (data-theme attribute present), not when it was chosen implicitly. So now that the last rule not related to the theme switcher was moved away from assets/scss/components/_dark-mode.scss, maybe rename it into _theme-switcher.scss?

[reuixiy]

Nice approach!

Maybe rename it into _theme-switcher.scss?

Agreed. And we can delete the related code in partials/utils/content.html now.

Also, I think we should add a "BREAKING CHANGE" note for this, since the way to customize the style of code highlighting has changed.

[palant]

All done.

[reuixiy]

Yep, except that we didn't update the annoying resources folder with example config.toml for themes.gohugo.io.

[palant]

Done.

[reuixiy]

Thanks!