Add dark theme/night mode to the website

[DamianArado]

The website already looks great. I know it's kinda cliché but it would be nice to have a night mode/dark theme here as well.
Thoughts?

[pnijhara]

This theme provides functionality for dark mode. But as far as I know, you can use only one mode at a time.
This config file can help → https://github.com/themefisher/bigspring-hugo-startup-theme/blob/master/config.toml
Feel free to play with config.toml file. If you find something more catchy do send a PR.