rounded corners css help please/code

trying to do rounded corners with css as mentioned here (or similar if someone can advise). able to do it with new, test file (html/css) for only one area that runs entire width of page but not able to figure it out incorporating into my existing external css (using float columns) for my site. here’s my css (question follows after code–thanks for bearing with me!):

[code=html]body
{
margin: 0 5 10 5;
padding: 0;
color: #191970
font-size: 90%;
font-family: arial, helvetica, sans-serif;
background-color:#8FBC8F

}

#header
{
width: 80%;
height: 5px;
background-color: ;
margin-bottom: 5px;
margin-left: 20%;
}

#gutter
{
float: left;
width: 3%;
height: 1px;
}

#col1
{ height: 500px;
float: left;
width: 25%;
background-color: #FFFACD;
margin-bottom: 10px;
margin-left: 3%;
}

#col2
{
float: left;
width: 45%;
background-color: #FFFACD;
margin-bottom: 10px;
padding: 5px;
}

#col3
{ height: 100px;
float: right;
width: 20%;
margin-right: 2%;
padding: 1em;
background-color: #FFFACD;
margin-bottom: 5px;
}

#col4
{
float: left;
width: 43%;
background-color: #FFF0F5;
margin-bottom: 10px;
margin-left: 4%;
padding: 5px;
}

#footer

{
margin-bottom: 10px;
margin-left: 21%;
padding: .5em;
float: left;
width: 55%;
height: 65px;
background-color: ;
}

h1
{

color: #191970;
font-size: 100%
margin: 0;
padding: 0px;
text-align: left;
}

h2
{ font-size: 125%;
color: #FFF0F5;
margin-top: 0;
padding-top: 3px;
text-align: center;
}

h3
{ color: #191970;
text-align: center;
font-size: 60%;
}
h4
{ font-size: 100%;
color: #191970;
margin-top: 0;
padding-top: 3px;
text-align: center;
}
p
{ font: geogia;
font-size: 10 pt;
color: #191970;
text-align: left
}

ul.square {list-style-type: square; color: #191970; font-size: 10 pt; text-align: left; }
ul.disc {list-style-type: disc}
[/code]

also wondering, is there a difference/advantage to using #/ div id= in css/html or ./ div class=? seems the same to me, except when mixing/matching id/class and #/., get some unusual effects.

thanks for help

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