[Feature] Use system preferences for dark mode

[veksen]

The dark mode should default to user preferences, if set, otherwise dark.

main relevant file: https://github.com/the-programmers-hangout/website/blob/master/src/ThemeProvider.tsx

can use window.matchMedia('(prefers-color-scheme: dark)')

[praveeno]

Hello @veksen
if someone didn't start working on this, I'll submit a PR for this.
Thanks

[veksen]

Sure thing!