Dark Theme for heroku.com

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku website

### Tell us about what you're trying to solve. What challenges are you facing?

There is no impact for not having this but it provides a better customer experience. Currently heroku.com is a lot of white and it can be a strain on the eyes. It would be nice to have a dark theme available when I visit heroku.com. Similar to how github.com provides the ability to select different themes. A lot of the development community likes the ability to change the appearance of tools and websites they use a lot.