@Dogra, Aman (Nokia - IN/Noida) The customization settings are available in the documentation with some samples to set style options in some scenarios.
The default style settings are defined here, which you can set through style options. For example,
const styleOptions = {
sendBoxBackground: 'rgba(0, 0, 255, .1)',
sendBoxHeight: '50',
sendBoxBorderLeft: '100',
sendBoxBorderRight: '50',
sendBoxBorderTop: 'solid 1px #E6E6E6'
};