First, log in WHM as either a reseller or system administrator.
Next click on Branding from the left column menu, right under the cPanel section.
You will see different themes available for edit.
Click on the theme will bring you to edit interface with a video tutorial.
The default theme is x3 but it can be different depends on hosting plan configuration.
Each theme will have different styles; you might want to apply theme edit to all themes/styles available.