1.Login to your blogger dashboard--> layout- -> Edit HTML

2.Scroll down to where you see </head> tag .

3.Copy below code and paste it just before the </head> tag .

<style type='text/css'>
.formbutton{
cursor:pointer;
border:solid 4px #ccc;
background:#999;
color:#fff;
font-weight:bold;
padding: 1px 2px;
background:url(http://3.bp.blogspot.com/_ar9PeTUrwMY/SuunWOFRlRI/AAAAAAAAAzo/sxOoBETSuD8/topnav_stretch.gif) repeat-x left top;
}
.formbox{
border:solid 5px #000000;
background:#ffffff;
color:#000;
font-weight:normal;
padding: 1px 2px;
}
</style>

NOTE:You can change background image,colors,borders,.. as your choice.

4.Now save your template.

5.Go to Layout-->Page Elements and click on "Add a gadget".

6.Select "html/java script" and add the code given below and click save.

<form id="searchthis" action="http://YOURBLOG.blogspot.com/search" style="display:inline;" method="get">
<input id="b-query" class="formbox" maxlength="255" name="q" size="35" type="text"/>
<input id="b-searchbtn" class="formbutton" value="Search" type="submit"/>
</form>

Note:Replace "YOURBLOG" with your blog name.

You are done.It will look like this:

CSS Search Box to blogger