Add dark theme for Swagger

[theletterf]

This adds a dark theme SCSS file for Swagger UI, adapted from https://github.com/Amoenus/SwaggerDark

Demo:

Fixes #2293

[xuchen]

thank you! this works for me perfectly!

[theletterf]

@chalin @LisaFC PTAL

[chalin]

Thanks for this @theletterf! A few comments:

• Some styling / formatting seems broken in your sample, or is that normal?
• We need a test of this either from the Docsy Example or in this repo. 🤔
• I'd prefer a deeper integration with Bootstrap and it's CSS variables, but this is better than no dark mode support.

[chalin]

@theletterf - a few more comments:

• Please address the build failures (maybe by running fix:format).
• Might there be a way that we can use the style(s) from https://github.com/Amoenus/SwaggerDark as is, and provide a delta over that. It might be easier to keep up with fixes to SwaggerDark.