Expose theme name in webviews

[mjbvz]

Fixes #97663

Adds a data attribute data-vsccode-theme to webviews with the current theme name. This is lets extensions write theme specific css for webviews, for example:

body[data-vscode-theme-name="One Dark Pro"] {
    background: red;
}

[aeschli]

There are hundreds of existing themes out there and they can be updated anytime. I don't see how a webview provider could keep track with that.
It were better if views don't define theme specific colors, but that an existing color of a theme is used (or a derivative of it (darker/lighter/mix).

Can you explain the use case?

[Binaryify]

here is the origin issue, Binaryify/OneDark-Pro#388
and this is the feature request #97663
I want custom markdown preview color highlight to make markdown style have more consistent with theme, so I write some css with the "markdown.previewStyles" in the package.json, but then I found it also actived in other themes too, this cause some problems, and don't have any way to limit that

Here's a comparison with custom markdown preview

[aeschli]

Ok, I see a theme wants to style the markdown preview through markdown.styles.
@mjbvz If it's just for that background color, can't the Markdown define a new color so that all themes can easily theme it? That's more in the spirit of theming.

[Binaryify]

@aeschli no just for background color, include the code highlight and the elements style

[mjbvz]

@aeschli Extensions should use the normal theme color variables in almost all cases. We also already expose the theme kind (light, dark, high contrast)

The theme name is helpful if you want to write CSS that is only enabled in one specific theme. For example, the official GitHub theme may contribute CSS rules to the markdown preview that are only enabled when the GitHub theme is active