Button Colors

This is a nice plugin that I really liked, but have a couple of problems.

I am using a shortcode, when changing the button background color, it did not work, but I was able to overcome this with CSS code.
Also the hover color, I had to look into the code and change it.

It is good that if you can add or fix what is necessary.

On the other hand, what I need your help with is that even though in WP editor and centered the shortcode, but the BMI calculator is not centered, I tried different ways including CSS, but did not get it to center.

How would I center it in CSS?

Thanks,