Step 1. Login to your cPanel account - The following article will help you How to log into your Client Area, cPanel and Webmail.
Step 2. Under Preferences section, please click the Change Style.

Stem 3. On the next page, there are several styles available to choose from. Hit Apply to enable your prefered style/theme.
Stem 4. Once done, cPanel style will be updated automatically.
Note: changing your control panel theme does not change any functionality, only the way it looks.