How to solve CSS errors in Blogger template using Firebug

Knowing CSS can help you in solving Blogger template errors ,or make some customization . But how to do that ? how to find where is the trouble or find what CSS attribute you need to change when  make customization ? In this post ,I will show you what I did : using Firebug (a Firefox add-on ) to work with CSSFirebug is a great add-on of Firefox browser ,it's very popular in web development