Custom Style Sheet being ignored?

I’m trying to tweak the display of some webarchives by setting a Style Sheet in the Web section of Preferences, but it is having no effect. If I edit the source of the webarchive and put the CSS directly in, it works.

Does the Style Sheet not get used for webarchives? Does the user style sheet not have highest precedence? Is there something I’m doing wrong? Thanks.

Actually I can’t tell for sure because that’s completely handled by the WebKit but I guess that the WebKit tries to reproduce the original layout of the webarchive.