You can change the default login theme for users which displays when they log into cPanel, WHM, and webmail.
1. Login to your WHM Root Account.
2. Go to Server Configuration >> Tweak Settings or type Tweak Settings in the search box and then the option will appear. Click on it.
3. Under Tweak Settings, click on the Display Tab and then choose the theme you wish to activate.
4. Scroll down to the end of page and click the Save button.
The new login theme will be applied successfully. You can verify it by visiting your cPanel, WHM or webmail login page.