Let’s assume that the name of your WHMCS theme is ‘RockHost’.
1. Open FTP and go to WHMCS root directory.
2. To make sure that you are at the right location, check if the current location contains templates, templates c directories.
3. The theme bundle will have RockHost folder which contains Templates folder in it along with some PHP files.
4. You have to upload the Templates folder and PHP files in WHMCS root directory, and overwrite all.
5. Go to WHMCS administrator panel now.
6. Click on Setup > General Setting, and set the Template value to RockHost.
Make sure that you add the correct name in all the three files.
Make sure that you add the correct page name in all the three files.
In case you want to modify the CSS that are applied by default, then customize them inside the /assets/css/custom.css file. Doing this will include the file after styles.css, which will allow you to override the CSS defined within it. Moreover, it won’t be affected in future when you apply updates to WHMCS software.
It is strongly recommended to add your custom CSS rules to custom.css file rather than editing the provided css files directly.