Add Shikiji css-variables theme to MarkdownOptions

[secondmanveran]

I'd like to add a css-variables theme via the newly added option to my vitepress configuration.

I created the css-variables theme and loaded it in the Highlighter instance:

import { createCssVariablesTheme, getHighlighter } from 'shikiji';

const cssTheme = createCssVariablesTheme({
  name: 'css-variables',
  variablePrefix: '--shiki-',
  variableDefaults: {},
  fontStyle: true,
});

const highlighter = await getHighlighter();
await highlighter.loadTheme(cssTheme);

But I'm unsure how to set it to my markdown config correctly.