Joomla is pretty good CMS but, is there any way to create custom template which will be joomla supportable ?
If you mean a page-specific template, then this is answered here: http://docs.joomla.org/How_do_you_assign_a_template_to_a_specific_page%3F
It describes the following steps:
In Joomla! there is a default template, but you can assign other templates to specific "pages" that are defined by menu links.
To assign a template to a page, you must first make sure that there is a direct menu link to the page.
- Go to Extensions>>Template Manager
- Select the Template and click the edit icon (or click the template name)
- In the left column, change "None" to "Select from List."
- Select the links you want to apply the template to.
- Save
Note that you cannot assign the default template to individual pages.