fix(background): avoid to cover the other plugin's dom, eg grid-line

[TZZack]

Background

There is a situation that if grid-line behind background plugin, the background plugin's dom will cover the grid-line plugin's dom and then the grid-line will not be seen.

Fix

This PR will set zIndex attr of background plugin's dom to -1 so we can make sure that the background's dom is at the back of same level