Как вставить код html на всех страницах не изменяя каждую страницу?

/* For IE & Mozilla-based browsers: */
 
.LHS {
  margin-left: 2.5mm;
  float: left;
  clear: left;
}
 
.RHS {
  margin-right: 2.5mm;
  float: right;
  clear: right;
}
 
/* For Mozilla-based (CSS2-fully complaint) browsers only: */
 
[class~="LHS"] {
  margin-left: 5mm;
}
 
[class~="RHS"] {
  margin-right: 5mm;
}
 
 
/* ###### Body Text ###### */
 
#bodyText {
  line-height: 1.5em;
  margin: 10mm auto 0 12em;
  padding: 0 1em 1ex 1em;
}
 
#bodyText p {
  padding-bottom: 2ex;
}
 
.topOfPage {
  font-size: 90%;
  font-weight: bold;
  font-variant: small-caps;
  text-decoration: none;
  padding-left: 1ex;
  padding-right: 1ex;
  float: right;
}
 
.smallCaps {
  font-variant: small-caps;
}
 
body {
  background-color: white;
  color: black;
  font-family: verdana, tahoma, helvetica, arial, sans-serif;
  font-size: 95%;
  margin: 0;
}
 
h1, h2, h3 {
  font-family: "trebuchet ms", tahoma, sans-serif;
}
 
h1 {
  font-weight: bold;
  font-size: 115%;
  border-bottom: 1px solid;
  padding-bottom: 0.5ex;
}
 
acronym {
  background-color: inherit;
  color: teal;
  border-bottom: 1px dashed teal;
  cursor: help;
}
 
code {
  font-family: "lucida console", monospace;
  font-size: 90%;
}
 
img {
  border: 0;
}
 
q {
  font-family: tahoma, verdana, helvetica, arial, sans-serif;
  font-weight: bold;
  border: 2px dotted;
  margin: 0 1em 1ex 1em;
  padding: 0.5ex 0.5ex 0.5ex 1ex;
  width: 9em;
  float: left;
}
 
 
/* ###### Header ###### */
 
#header {
  border-bottom: 2px solid;
  height: 5em;
  margin-bottom: 5mm;
  padding: 1em 2.5mm 0 5mm;
}
 
.headerTitle {
  font-size: 250%;
}
 
.headerLinks {
  text-align: right;
}
 
.headerLinks * {
  text-decoration: none;
  font-weight: bold;
  padding-right: 1ex;
}
 
.headerLinks a:hover {
  text-decoration: underline;
}
 
 
/* ###### Side Box ###### */
 
.sideBox {
  border-top: 2px solid;
  border-bottom: 2px solid;
  width: 10em;
  margin-top: 5mm;
}
 
.sideBox div {
  font-weight: bold;
  border-bottom: 1px dashed;
  padding: 0.5ex 0.5em 0.5ex 0.75em;
}
 
.sideBox a, .sideBox a:hover, .sideBox span {
  color: black;
  text-decoration: none;
  line-height: 1.25em;
  display: block;
  padding: 1ex 0.5em 1ex 0.75em;
}
 
 
/* ###### Footer ###### */
 
#footer {
  color: inherit;
  border-top: 2px solid;
   padding-bottom: 0.75ex;
  clear: left;
}
 
.footerImg {
  padding-right: 3mm;
  padding-top: 2mm;
  float: right;
}
 
#footer div {
  padding-left: 3mm;
}
 
#footer * {
  background-color: inherit;
  font-size: 92%;
}
 
#footer a:hover {
  text-decoration: none;
}
 
.footerCol2 {
  position: absolute;
  left: 7em;
}