Fix theme styles and selector causing conflicts with Designa

[alex-ketch]

These changes have been sitting around for quite a bit awaiting some of the fixes in stencila/designa#26.

Seeing these themes in action on /Open, I want to reevaluate some of the decisions and structures around the selectors and how we're writing the themes. My current hunch is that we might want to establish rules of targeting only block level elements which are direct descendants of an <article>. Though the details need to be ironed out regarding how to deal with child elements that we want to style as well, such as ol > li > figure.

[nokome]

I'm a little uncomfortable with this use of custom selector for a non-semantic pre-formatted element. It feels a little ad-hoc. All the other uses of custom selectors below relate to an itemtype, an itemprop or containers for itemprops (e.g. authors). Looking below at the use of :--pre, it seem that to be consistent with that, we need a @custom-selector :--CodeBlock [itemtype='https://schema.stenci.la/CodeBlock']; and for Encoda to encode an itemtype for CodeBlock elements.

Generally, I think we should:

• decide which Schema node type we are not going to add itemtype to in the HTML encoding (adding to p, li, strong etc is probably over the top)
• in Encoda add itemtype to HTML encodings of all other node types
• in Encoda, or here, autogenerate the @custom-selectors for the types and properties

[nokome]

As part of general housekeeping, I'm going to close this PR under the assumption that it is now out-of-date. Of course, feel free to re-open it.