Use the Design > Basics tab to change elements like your site banner; how your thumbnails are displayed; which fonts and colors to use; and so on.
Changes you make in this section never destroy the content of your site -- they just change the way your site content is displayed. And changes you make are specific to the theme you've selected, so if you change themes you can get an entirely different look.
This short seven minute video can help orient you to these features.
For more information on working with themes, see the Themes Tab help page.