Changing themes in e107 is quite easy. You should log in as admin and go to Theme Manager. Once there, you will have a choice of several predefined themes which you can use for your e107 application. When you choose a theme, you should click on Set As Site Theme to change the theme for your site. Some themes also have a Set As Admin Theme button, which, as the name suggests, will change the theme of your admin area:
You can easily install more themes by uploading the zip file with the theme in the www/e107_themes folder. Then you should extract the archive using the File Manager in your cPanel. Once the theme archive is extracted, you can set the theme as a default one for the admin area or for the whole site.