You don't need to customize any core Moodle code... or even your theme. Moodle has all the functionality you need built right in!
First start by going to:
Site Administration > Users > Accounts > User profile fields
This is where you create new sections and fields for the profile page.
If you want to add a new field, select the type from the "Create a new profile field" dropdown list.
New fields will be added to the "Other fields" section of the profile page by default. If you want to create a new section on the profile page, click the "Create a new profile category" page.
Site Administration > Appearance > Additional HTML
You can add STYLE and SCRIPT tags on this page. They will be inserted into your page header or body depending in which field you use.
Hope this helps.