Edit your config.xml file and add an entry for your own skin
Code:
<skins>
<skin folder="default" name="default" />
<skin folder="yourSkinFolder" name="yourskin" />
</skins>
Then create a yourSkinFolder folder in installFolder/skin.
Copy the whole content of installFolder/skin/default into this newly created folder and modify the css files as wanted. You can also change all the gif/jpg images.