How To Customize My Website

Weebly has a couple of options that would allow me to customize my website.  In the "elements" tab I can drag "custom HTML" into my page and edit HTML right there.  If I want to customize the CSS I can do that under "design" and then "Customize CSS".  The original CSS pops up and I have the option of writing in my own/changing what they have or I can press several options to the left of the page which change the CSS slightly and I can see the effects of the change immediately in the page viewer in the lower portion of the screen.