style(docs): use github-dark hightlight theme

Use the `github-dark` highlight theme for fence blocks in vitepress,
instead of the default `material-palenight` theme.

This increases the contrast ratio of `#comments` from 2.75:1 to 4.43:1,
which is a lot more visible.

It still doesn't reach WCAG 2.0 level AA contrast standards,
which requires 4.5:1 as a minimum constrast ratio, but 4.43:1 is
pretty close, and we don't need to manually modify the theme's colours.

packages/mermaid/src/docs/.vitepress/config.ts

@@ -4,6 +4,8 @@ import { MermaidMarkdown } from 'vitepress-plugin-mermaid';
 import { defineConfig, MarkdownOptions } from 'vitepress';
 
 const allMarkdownTransformers: MarkdownOptions = {
+  // the shiki theme to highlight code blocks
+  theme: 'github-dark',
   config: async (md) => {
     await MermaidExample(md);
     MermaidMarkdown(md);