Customize the theme to align with Plone branding

[stevepiercy]

The primary navbar needs some love. I'm not sure why the title and logo are not stacked.

[nileshgulia1]

@stevepiercy I can take this.

[nileshgulia1]

hm... looks good in https://6-dev-docs.plone.org

[stevepiercy]

This turned out to be a configuration issue in conf.py from not copying over the required CSS:

html_static_path = [
    "_static",
]