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To add border use style={{border:your color}}
For example
<Button style={{border:red}}></Button>
If you want to make a circle that will work on any device the only thing that you should do is to give the same height and width the same value and then give the borderRadius a really high value I personally give it 1000 so it will big enough for most of the cases
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circle :{
height : 30 ,
width :30,
borderRadius: 1000,
}