Test theme hint in previewHtml

[jrieken]

tests #7589

• any os - @weinand

When previewing html we now indicate the active theme group. Those are vscode-light, vscode-dark, and vscode-high-contrast. Make sure one and only one of them is always set on the document. Switch another theme and make sure it updates

[weinand]

tested by previewing a markdown file with this contents:

<script language="JavaScript">
  document.write(document.body.className);
</script>