Fix a regression after e7cdcc0d, which
added the client's standard CSS reset to the annotator CSS bundle.

The font-family used by the adder buttons was changed from the user agent's
default for `<button>` elements to `inherit` by this reset. The result
was that the font family would vary depending on the web page.

Fix this by adding a `:host > *` selector which sets style properties for all
shadow roots created by the annotator. This is the equivalent of the `body`
selector in sidebar.scss.