twbs--bootstrap/scss/mixins/_forms.scss

// Form control focus state
//
// Generate a customized focus state and for any input with the specified color,
// which defaults to the `@input-border-color-focus` variable.
//
// We highly encourage you to not customize the default value, but instead use
// this to tweak colors on an as-needed basis. This aesthetic change is based on
// WebKit's default styles, but applicable to a wider range of browsers. Its
// usability and accessibility should be taken into account with any change.
//
// Example usage: change the default blue border and shadow to white for better
// contrast against a dark gray background.
@mixin form-control-focus() {
  &:focus {
    color: $input-color-focus;
    background-color: $input-bg-focus;
    border-color: $input-border-color-focus;
    outline: none;
    @include box-shadow($input-box-shadow-focus);
  }
}