Joomla is pretty good CMS but, is there any way to create custom template which will be joomla supportable ?
If I understand your question you want to convert an existing template that isn't even a Joomla template. If that is the case I found it very hard to do, of course depending on where the template was coming from. I ended up taking a template for Joomla 1.5 and starting from that. (Although the ja_Purity is a messy one when it comes to the CSS, looking back I would should have started with another one....)
Yes, you can do it. Copy default template which comes with Joomla and customize it, and you can install it now.
Refer : http://docs.joomla.org/Joomla!_1.5_Template_Tutorials_Project
Follow the SiteGround tutorial.
1st result for Google: joomla change template.
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.
来源：https://stackoverflow.com/questions/2876322/custom-template-support-for-joomla