Little visual difference between H1 and H2 headers

[choldgraf]

Currently there's very little difference between H1, H2, and H3 headers. This is most important for H2 and H3 since they often make up the major parts of the page. In addition, the headers are only distinguishable by a larger font size which can make them blend in with the text a bit.

Here's an example of similar headers in other themes:

Furo:

Docusaurus:

Material mkdocs:

[choldgraf]

Have a slight tweak to the header CSS to try and address this in:

• https://github.com/executablebooks/sphinx-book-theme/pull/687