Feature request: Change to dark theme/night mode automatically based on user preference

**Is your feature request related to a problem? Please describe.**
I just realized today that cyberchef has dark theme support , I'm kinda used to apps just selecting the theme based on your device and/or browser theme preferences so I never realized that there's a theme setting in the options.

A lot of people are probably like me, they use this tool in the dark and don't visit the options and they get this bright website. Spare their eyes and implement this.

**Describe the solution you'd like**
Implement this:
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme
It's supported by all the browsers.
https://caniuse.com/prefers-color-scheme

**Describe alternatives you've considered**
There's also this but its experimental and not supported by all browsers:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Color-Scheme
https://caniuse.com/mdn-http_headers_sec-ch-prefers-color-scheme

**Additional context**
None