The bsStyle prop, available in many components in React-Bootstrap, is used to map to a Bootstrap class for styling; for example, the Bootstrap class used for Button is `btn-${bsStyle}`. Use bootstrapUtils to create a custom class that is used in lieu of the classes provided by Bootstrap:

bootstrapUtils.addStyle(Button, 'custom');

const customButtonStyle = (
    <style type="text/css">
    .btn-custom {
        background-color: purple;
        color: white;
    <Button bsStyle="custom">Custom</Button>