Don't turn on dark theme based on time

[kidonng]

I think we are at a stage where we can assume prefers-color-scheme and system wide dark mode are widely supported: https://caniuse.com/prefers-color-scheme

Therefore the following lines:

gitstalk/src/App.vue

Lines 72 to 73 in 2ff5fd5

	today.getHours() >= 22 ||
	(today.getHours() >= 0 && today.getHours() <= 5) ||

Which turns on dark them when it is 22:00-05:00 seems unnecessary. As for myself, I frequently encounter a situation that Gitstalk turns dark while rest of my system doesn't, which is pretty confusing.