Fixed html attributes not showing in design settings preview
Fixed html attributes not showing in design settings preview

no issue

- by using `innerHTML` we were missing the lang and class attributes on the `<html>` element that are added by themes, breaking things like dark mode
- switching to `outerHTML` includes the `<html>` element and is handled automatically by iframes when writing to the `contentWindow`

TryGhost/Ghost-Admin,TryGhost/Ghost-Admin