Utilize "prefers-color-scheme" to set default theme

[crutchcorn]

Many users set their light/dark mode theme in their operating system.

By using prefers-color-scheme, we can set the default site theme to match with the expected theme.

Windows 10/11, macOS, Android, iOS, and many Linux distributions all support this functionality

[crutchcorn]

Oh, apologies - it seems that this is currently supported on first page load. However, on subsequent page refreshes (even if manually not set), it doesn't change.

Might be nice to have detection to see if manually set or not