To create a new skin, go into directory 'htdocs/theme/' and copy for example all the directory 'eldy' into a new directory with the name of your skin.
In this directory, change name of file *.css* to match name of directory.
Edit file AUTHOR with your information.
Now, edit the CSS file (*.css*), and all images inside the img directory to personalize your skin.
The new skin will appear automatically in the list of available skins in page Setup - Display. This will allow you to activate it by default, or in Users and Groups - Display tab to activate it for a particular user.
To test your theme on all pages without activating it, just specify it in the address bar by adding the theme parameter.
Once you have created a new skin, you can build a package to distribute it by running the script build/makepack-dolibarrtheme.pl
You will get a file .tgz that you can submit on the Dolibarr download area (Category contrib).