Check out example codes for "delected blue border when an input is selected". It will help you in understanding the concepts better.

Code Example 1

input:focus { 
    outline: none !important;
    border-color: #719ECE;
    box-shadow: 0 0 10px #719ECE;
}
textarea:focus { 
    outline: none !important;
    border-color: #719ECE;
    box-shadow: 0 0 10px #719ECE;
}

Code Example 2

.input:focus {
    outline: none !important;
    border:1px solid red;
    box-shadow: 0 0 10px #719ECE;
}

Code Example 3

//SCSS format

input{
  &:focus { 
    outline: none !important;
    }
}
textarea{
  &:focus { 
    outline: none !important;
    }
}

Learn ReactJs, React Native from akashmittal.com