Change the default color of debug log

[waynexia]

The color of debug log defaults to deep blue, which is hard to read on a dark background:

We can change to some brighter color, or only add color to the level text, like this:

[zyy17]

@waynexia Seems reasonable, and let me try to improve it. Since the different background colors of the terminal, I think the better way is to detect the background and apply the compatible color in different background colors.