I am on 9.2.1 now and gee... too easy... :p
 
Just do the following, make sure you back up every files that you will be modifying:
 
First do 
mount -uw / so you can write to the file system
 
Then goto the the following folder:
cd /usr/local/www/freenasUI/static/css
 
Change the following lines
 
In
 layout.css
	
	
	
		HTML:
	
	
		//Change the height from 72 to 0.
#page-header {
    height:0px;
    background: #464C53 url("../images/headerbg.png") repeat-x;
}
 
// Add the following after
#page-header div img {
    width: 0px;
}
 
// Change the height from 80 to 0
#footer {
    height:0px;
    background: #464C53 url("../images/footerbg.png") repeat-x;
}
// Change the height of topLayout from 125 to 55
#topLayout {
    height:55px;
    overflow:hidden;
}
// Change the height of footerLayout 0
#footerLayout {
    height:0px;
    width:100%;
    padding-bottom:0px;
}
	 
 In 
custom.css
	
	
	
		HTML:
	
	
		<!-- Update, i didn't like how the message would get cut off. I put the notification message box to bottom right instead -->
#messages {
    position: absolute;
    bottom: 5px;
    right: 5px;
    min-width:250px;
}
<!-- Comment out the margin-left of .success -->
<!-- Update, also commenting out the height -->
.success {
    background-color:#40664D;
    min-width:400px;
    /*margin-left:100px;*/
    border:1px solid #222;
    max-width:500px;
    word-wrap:break-word;
    color:#000;
    /*height:50px;*/
    text-align:center;
    opacity:0.6;
    filter:alpha(opacity=18);
}
	 
 The above should take care of all the css. Hope I didn't missed any... Now lets go to move 2 divs to another location and you are done...
 
cd /usr/local/www/freenasUI/templates
 
Comment out the following lines
In
 base.html
	
	
	
		HTML:
	
	
		                <!-- Comment out these 2 dives
                <div id="messages"></div>
                <div class="clear"></div>
                -->
	 
  
Update: I don't like how the notification message got cut off after my change, so I moved it to lower right corner area instead.
In 
base.html again, put it before the body end tag
	
	
	
		HTML:
	
	
		    <div id="messages"></div>
    <div class="clear"></div>
 
</body>
	 
  
 
Control + F5 your browser and enjoy. This completely hide the top and bottom bars, and kept all the functionality intact.
 
PS: I think if you reboot, it might get override, I vaguely remember back in 8.0, i had to modify the files somewhere else... but I couldn't find exactly where the "original" files is stored.