How create a theme in concrete5

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Hello , Im new to CMS Concrete5 , i purchase temeplete , and i would like to use Concrete5 to buid site , I did seacrh In the net , how to create a theme in concrete5 . i fount this :
1. Create a new folder in themes directory.

2. Create there a plain text file description.txt with only 2 lines: the first should have theme title, the second line contains a short description.

3. Create a ‘thumbnail.png’ file with 120×90 thumbnail for the skin.
I didn't understabnd this line
should i create default.php in theme Folder what header is ?? is it the file of the front page of my tempelate ????
4. Create ‘default.php’ file with the xhtml or html code of your skin. For the editable area, use the following code snippets:

<?php
$ah = new Area('Header');
$ah->display($c);
?>

Any example simple , any help
Thank you
Simon