the layout of Google API theme looks quite similar to docfx's

+1
The Material Theme for MkDocs could be used as the base for the docfx theme
https://squidfunk.github.io/mkdocs-material/
There are already a couple of ports of that theme to other generators:
Creating a theme here shouldn't be a concern of the core project, rather simplifying the creation of themes should be.
There are serious markup, css styling and accessibility issues with the default templates and I would suggest getting someone with UX/frontend skills involved to help clean it up.
I've noted in an attempt to style the default output:
And that's just scratching the surface.
This work would require a major version as it would introduce breaking changes.
I'm more than a little disappointed that MS didn't strive to make the output more inclusive.
@JimBobSquarePants,
We know there are lots of areas to improve in docfx and creating a customized theme is just one of the outstanding problems. Like you mentioned, a major breaking change is needed to modernize the system. We had some initial discussions around how to build the next version of docfx and customized themes is definitely a big topic. It is still at the very early stage, but we'd like to hear all food for thoughts.
Hey @yufeih long time no chat! 馃槃
When you get around to making plans for vnext more concrete I'd love to help so please let me know.
Most helpful comment
The Material Theme for MkDocs could be used as the base for the docfx theme
https://squidfunk.github.io/mkdocs-material/
There are already a couple of ports of that theme to other generators: