View source for Explain The Template Manager (Site and Administrator) , Installing a new template (Site and Administrator) and Deleting a template (Site and Administrator) , Assigning a template (Site)

Templates allow you to define the look and feel of your Joomla website.
First, go to http://localhost/joomla15/administrator and log in with the username “admin” and the password you specified during the installation. If you are already logged in, you do not need to do this again.
From the “Extensions” menu, select “Template Manager.”
The template selected with the star is the current template. To change to a different template, select the template you wish to use and click on “Default.” For the landscaping website, the “rhuk_milkyway” template will fit well:
Click on “rhuk_milkyway” in order to edit some of the settings for the template. Because this is a landscaping website, let us change the colour scheme to green.
Take a look to see how the look and feel of the site has changed.
Next, on the edit page, click on “Edit HTML” in order to adjust the template itself.
Remember when we put modules in? 
Each module was placed into a specific section. These sections are defined in the template. For example, the “top” position is defined by this code: <jdoc:include type="modules" name="top" />.