Hi Qin,
1. Not centered. => this is default
2. The direction of the icon and text is wrong, it should be the icon on top. => its not wrong. its different then what you want
3. How do I change the color of icons and text. => See: https://atlas.mendix.com/p/getstarted/7881299347898769
4. How to change the background color of the entire bar => See: https://atlas.mendix.com/p/getstarted/7881299347898769