This tutorial will show you how to change the parameters of the default Joomla template
1) Go to Extensions...

2) ...Template Manager
Â
3) Click on the default theme

Look to the right side of the page
Â
Here, you can change several aspects of the default theme

Color Variation will change the border color of all the main menus to the color specified
Â
4) Choose a new color

Background Variation will adjust the main background color for the page

The last option, Template Width will determine how Joomla fills up the window
Â
By default, Joomla is set to Fluid with maximum; this means Joomla will stretch to fit the window, to an extent, but not wider than a certain amount
Â
Medium and Small will both force Joomla to be a fixed width at all times, regardless of how wide the window is stretched
Â
The last option is Fluid (with no maximum)
Â
5) Select a new template width behavior

6) Click Apply
 
The page will refresh, allowing us to come back and change the settings if we don't like the way they look
Â
Let's check out the new settings
Â
7) Click Preview
 
Success! The main menu borders have been updated to black and so has the background color. Also note that Joomla has fluidly stretched to fill the browser window

Â
This is the end of the tutorial. You now know how to customize the default template in Joomla