Hi - What version are you using? Have you found and reviewed some of the material that already exists related to this in training or other posts? For example, check the Related Posts when viewing this post. This one below is pretty detailed. I am using 7.2.1 currently and did a quick test of downloading, editing, and then uploading a new version of pyApplicationProfile. I just did a private edit of the pyApplicationProfile Word Template, the changes made were reflected when I ran the document (I just made some simple changes).

https://docs-previous.pega.com/implementation-methodology-and-dco/dco-61-how-customize-application-document-templates

Can the content be changed - Can be achieved by saving pyCaseAssets rule in you application and and customize it as you need
Font and color of the headers be changed and
Can we add more content in the front page can be achieved by customizing your portal from where you can change the portal header(color, font, and can add images)
Standard text within the headers be updated - for this you need to create a mixin in skin rule.

Thanks

Jibran