I want to change the default index.html created with new domains.

Whenever a new User or new domain is created, they recieve a default index.html file in their public_html directory.
If you wish to customize this holder page that they will receive, you can edit the index file in the following directory:

/home/resellername/domains/default/index.html

where resellername is the name of the account which is creating the User (eg: "admin").

All files within this default directory (sometimes known as a skeleton directory) are copied to the User's public_html folder, chowned to that User. You can use an index.php or index.shtml file if you wish (or whatever you want), however only the index.html file (if it exists) will be tokenized. The following tokens are avaiable:

|DOMAIN|
|USERNAME|
|DATECREATED|
|IP|

Note that all files in the default directory must be world readable (755) becaues they're copied over by a process running as the new User. Any directory in the default directory will also be copied.