JSFiddle - React, Tailwind, and code Playground

HTML

<button id='start_timer_button'> Start Timer</button>

CSS

button.stop_button{
	background: red !important;
	padding: 5px 29px 5px 29px !important;
}

#start_timer_button{
	float: left; 
	background: #389838; 
	border: none; 
	color: white; 
	border-radius: 10px; 
	padding: 5px 10px 5px 10px; 
	font-weight: bold; 
	position: relative; 
	top: -2px; 
	right: -20px;
	opacity: .80;

	&:hover{
		opacity: 1;
	}
}

JavaScript

$('#start_timer_button').on('click', function(e){
    e.preventDefault();
    $(this).text('stop').addClass('stop_button');
});