If you want to make changes to your design then you can use Inspect Tool to see the source code of your website and the CSS rules applied to it.
To enable it right click and from the pop-up menu select Inspect Element For more details please visit Chrome inspect element or Firefox inspect element. Meanwhile searching in google about it will be helpful.
For example, we try to change bullet and add border bottom red color in sidebar Recent Posts widget. please watch this video tutorial: