Generated static HTML documentation looks barebones

Description

The generated HTML documentation looks pretty barebones, ideally it should look like the preview on https://editor.swagger.io/.

I generated the HTML documentation for the petstore example with those two commands:

$ docker run -v /tmp/openapi:/shared --rm -it openapitools/openapi-generator-cli:latest generate --generator-name html --output /shared --input-spec https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml --skip-validate-spec

petstore-html.tar.gz

$ docker run -v /tmp/openapi:/shared --rm -it openapitools/openapi-generator-cli:latest generate --generator-name html2 --output /shared --input-spec https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml --skip-validate-spec

petstore-html2.tar.gz

openapi-generator version

Whatever latest is on https://hub.docker.com/r/openapitools/openapi-generator-cli/tags.

OpenAPI declaration file content or url

The petstore example: https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml

Command line used for generation

See docker commands above.

Steps to reproduce

See above.

Suggest a fix/enhancement

Templates need to get some styling.

Run the documented Docker commands for the html and html2 generators with the linked Petstore OpenAPI example, then compare both outputs with the Swagger Editor preview. Inspect the templates used by those generators; done means the generated static documentation has styling comparable to the referenced preview.