If you are just looking to change the appearance of cPanel, I recommend just editing the style than the theme. Editing themes can cause many issues and is usually unnecessary for cosmetic changes.
If you have reseller or root access to the server, just login to the cPanel interface and click on the Branding Editor to begin the process of customizing the appearance of the cPanel interface. The most powerful editing tool is the CSS editor so you can modify the CSS used for displaying content on the page.
If you need help using the Branding Editor, let me know.