1- Log in to blogger

2- On your Dashboard, select Layout. This will take you to the Template tab. Click Edit HTML. Under the Edit Template section you will see you blog's HTML.

3- paste the CSS definition in the top of the template code:

...
<b:skin><![CDATA[/*
-----------------------------------------------
Blogger Template Style
Name:     Rounders
Designer: Douglas Bowman
URL:      www.stopdesign.com
Date:     27 Feb 2004
Updated by: Blogger Team
----------------------------------------------- */

#navbar-iframe {
   display: none !important;
}

/* Variable definitions
  ====================
   <Variable name="mainBgColor" description="Main Background Color"
             type="color" default="#fff" value="#ffffff">
   <Variable name="mainTextColor" description="Text Color" type="color"
             default="#333" value="#333333">
   ...