If you are planning to modify some of the part of the default template or any texts used on your site then, you can easily do it by editing template files or language file.

The default template file including default assets like images, CSS and JavaScript etc is located at /templates/default/.

And, the default language file is located at /inc/locale/en_US.php

For an example, to modify the text of homepage, you can modify the template file located at /templates/default/welcome.tpl