Why do the colors change in order when switching themes?

[rea1shane]

I want to add delay when theme transition, I add these CSS:

* {
    transition: color 200ms ease-in-out, background 200ms ease-in-out;
}

But I noticed that the color transitions of the elements are not simultaneous, but sequential, why?

To Reproduce:

1. New hugo site add set paper-mod theme.
2. Add above CSS.
3. Add a post.
4. Start server, enter the post and switch theme.

And I found #407, is it the best way to do this thing? It modifies a lot of files and I want to as few as possible.