For the style customization like this, you can use Google Chrome or Firefox inspect element. What I usually do is hover on any HTML element (i.e. button, tag, text, …) you want and right click on it and then click on inspect element, it will show you which CSS class it is using and which CSS file and the path to this element on the right side.
If you want to change in CSS, just add your custom CSS code into the file: root/templates/your default template/local/css/custom.css