I'm trying to target an element in the the DOM that has already been returned, and rendered, by another component; I was hoping to use a getElementById() call, and then change the CSS of that element.
Its bad idea to use getElementById in React. You can use style like this
<div style={{ height: 10 }}>
Hello World!
</div>
Or give attribute className to your element and make styles for that class in css.
<div className="button" />
You can use element.style to change css properties.
Edit : It seems that you shouldn't use getElementById with React.
let element = document.getElementById('hi')
element.style.color = "white";
element.style.backgroundColor = "black";<div id="hi">Hello</div>in the css file add
#h1 { font-size: '12px'; }
and in HTML file use this
<div id="hi"></div>