If using the following HTML:
<button id="submit-button"></button>
Style can be applied through JS using the style object available on an HTMLElement.
To set height and width to 200px of the above example button, this would be the JS:
var myButton = document.getElementById('submit-button');
myButton.style.height = '200px';
myButton.style.width= '200px';
I believe with this method, you are not directly writing CSS (inline or external), but using JavaScript to programmatically alter CSS Declarations.