[feature] Make light/dark mode responsive to system theme

**Version and OS**
0.40.0.4

**Is your feature request related to a problem? Please describe.**
When I was using changedetection.io, my system theme was Dark, but the website defaulted to its Light theme.

**Describe the solution you'd like**
changedetection.io should always default to the user's system theme. This default can be overridden whenever the user clicks the toggle switch.

**Additional context**
This can be implemented using the [`prefers-color-scheme`](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme) CSS attribute. [This post](https://www.section.io/engineering-education/watch-for-system-dark-mode-using-js-css) has more implementation examples, though I'm not sure what solution would best fit into the app.