Webview CSS: data-vscode-theme-name attribute changes when the language changes

[pwang347]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.68.0-insider
• OS Version: Windows 10

Steps to Reproduce:

1. Write webview CSS depending on data-vscode-theme-name, e.g. see support limit markdown.styles active in single theme #97663 (comment)
2. Change the language by installing a language pack - the CSS will stop working

Would it be possible to use an ID instead of a name string for this attribute (e.g. a new attribute to prevent breaking changes), or perhaps to force this attribute to be English only?

[pingren]

@pwang347 @mjbvz

I submitted a PR to address the issue. Please take a look when you have a moment.