When starting out, one of the first things people do is navigate to the core
/themes directory and take a look at the files in the themes to get an idea of the general structure and contents. Unfortunately, many people make the mistake of starting out by directly customizing core themes. Do not make this mistake! They usually run into roadblocks and frustration shortly thereafter. Drupal has a large and diverse user base, and the main goal of a core Drupal theme is to cater to the masses.
Do not modify core themes!!
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