Why header does not stretch all the way from left to right?

Question

I am using Dreamweaver CS3.  I am a newbie.  I modified a dreamweaver two column template.
Now I find that there is always a gap between the header and the left and right edge of the web page!  I am enclosing the css stylesheet.  Please tell me where I went wrong.  Thank you very much.
@charset "utf-8";
body  {
    font: 100% Verdana, Arial, Helvetica, sans-serif;
    background: #666666;
    margin: 0; /* it's good practice to zero the margin and padding of the body element to account for differing browser defaults */
    padding: 0;
    text-align: center; /* this centers the container in IE 5* browsers. The text is then set to the left aligned default in the #container selector */
    color: #000000;
.twoColFixLtHdr #container {
    width: 90%;  /* using 20px less than a full 800px width allows for browser chrome and avoids a horizontal scroll bar */
    background: #FFFFFF;
    margin: 0 auto; /* the auto margins (in conjunction with a width) center the page */
    border: 1px solid #000000;
    text-align: left; /* this overrides the text-align: center on the body element. */
.twoColFixLtHdr #header {
    background: #a4c2c2;
    padding: 0 10px 0 20px;  /* this padding matches the left alignment of the elements in the divs that appear beneath it. If an image is used in the #header instead of text, you may want to remove the padding. */
#logo {
    height: 71px;
    padding-bottom: 12px;
    width: 100%;
    background-color: #999966;
    padding-top: 12px;
#nav {
    height: 28px;
    padding-top: 12px;
    padding-bottom: 12px;
.twoColFixLtHdr #header h1 {
    margin: 0; /* zeroing the margin of the last element in the #header div will avoid margin collapse - an unexplainable space between divs. If the div has a border around it, this is not necessary as that also avoids the margin collapse */
    padding: 10px 0; /* using padding instead of margin will allow you to keep the element away from the edges of the div */
    font-family: Geneva, Arial, Helvetica, sans-serif;
    color: #000000;
    text-align: center;
    font-size: 36pt;
.twoColFixLtHdr #sidebar1 {
    float: left; /* since this element is floated, a width must be given */
    width: 40%; /* the actual width of this div, in standards-compliant browsers, or standards mode in Internet Explorer will include the padding and border in addition to the width */
    background: #EBEBEB;
    padding-top: 15px;
    padding-right: 10px;
    padding-bottom: 15px;
    padding-left: 20px;
    font-size: 14px;
    line-height: 21px;
.twoColFixLtHdr #mainContent {
    float: right;
    width: 42%;
    margin-top: 0px;
    margin-right: 0;
    margin-bottom: 0;
    margin-left: 0px;
    padding-top: 15px;
    padding-right: 10px;
    padding-bottom: 0;
    padding-left: 20px;
    font-size: 14px;
    line-height: 30px;
.twoColFixLtHdr #footer {
    padding: 0 10px 0 20px; /* this padding matches the left alignment of the elements in the divs that appear above it. */
    background:#DDDDDD;
    clear: both;
.twoColFixLtHdr #footer p {
    margin: 0; /* zeroing the margins of the first element in the footer will avoid the possibility of margin collapse - a space between divs */
    padding: 10px 0; /* padding on this element will create space, just as the the margin would have, without the margin collapse issue */
.fltrt { /* this class can be used to float an element right in your page. The floated element must precede the element it should be next to on the page. */
    float: right;
    margin-left: 8px;
.fltlft { /* this class can be used to float an element left in your page */
    float: left;
    margin-right: 8px;
.clearfloat { /* this class should be placed on a div or break element and should be the final element before the close of a container that should fully contain a float */
    clear:both;
    height:0;
    font-size: 1px;
    line-height: 0px;

Answer 1

.twoColFixLtHdr #header {
    background: #a4c2c2;
    padding: 0 10px 0 20px;  /* this padding matches the left alignment of the elements in the divs that appear beneath it. If an image is used in the #header instead of text, you may want to remove the padding. */
The instructions (comments) above give you a clue. Zero out the left (20px) and right (10px) padding.
.twoColFixLtHdr #header {
    background: #a4c2c2;
    padding: 0;