add dark mode support

Hi!

I'm sorry for the inconveninece. This is actually in my TODO for the next edition of the course.

Hi Alexander,

Thanks a bunch for all the great things you’ve done here! I know you mentioned a few browser extensions for this, but on other devices, we usually don’t have access to those. I was hoping you could prioritize the dark mode feature in the short term.

Please add Dark Mode, its really hard to read the interactive course :-(

Install this chrome plugin, it helps convert to dark mode 

https://chrome.google.com/webstore/detail/high-contrast/djcfdncoelnlbldjfhinnjlhdjlikmph