Steps to reproduce:

1. Create EuiControlBar that has button as one of the controls:

const controls = [{
	controlType: 'button',
    id: 'button',
	fill: true,
	color: 'primary',
    onClick: ()=>{},
}]
<EuiControlBar controls={controls}>

The button looks like in the picture below:

2. Expected:
   

Bug can be reproduced for all colors.