Demo On Creating a Theme Change

This is my blog before the theme change:

This is my blog after the theme change 

Official Support Page on how to do a theme change in Blogger Use themes to change how your blog looks. 

First: To apply a theme change you first need to log into Blogger.

Next: Find the "Theme" tab in the left hand menu: 

This will reveal the themes which are possible to select in the main window:

Find a theme that looks good and select it to find out more details:

The options available are to Preview #1, To Customize (add or remove components #2), or to simply Apply the theme #3. 

Special Note: Some themes might make sweeping changes to your site, and depending on which one you select you may be prompted to make a back up before you apply the theme.