With Customize options you can make changes in a single page or on the whole website. You will find these options in the left upper side of your site:

You can change  font type, size and color of the text on pages in Typography and colors section:

In Site Identity the following fields could be changed: site title, tagline, site icon.  You can also choose menu type ( fixed or not fixed), add copyright information and upload logo image: 

Blog page. With two options shown below, you can show or hide sidebar and information list in blog page:

In Albums page choose an appropriate albums style, column size, category, number of posts, post`s style and column size. You will have to select popup type for gallery and text for protected gallery:

Colors. Header text and background colors can be changed in this section:  

Header image as well as background image can be uploaded in the following sections:

It is possible to reorder menu items, add a new one and change menu locations in Menus section:

In Static Front Page you can choose the way front page will be displayed: 

Import Demo Data: this option was already mentioned when importing demo data.

Additional JS: this section was specially created for you to easily customize your site by adding JavaScript code: