A long time ago, a user asked this question, and someone answered the answer bellow, I took the answer and saved in a text file because I found it usefull, I do not know who wrote this answer, so I appologize to him that I am using the answer without referencing the author:

The core code allows you to override the DNN skin.  This is all that the DNN site skin is doing.  There are 2 ways to override the skin.  First, by a querystring.  This is the method that is used to preview the skins in the Page Settings, Site Settings, and Skin Manager.  The other way is to set a cookie.  This is the method that the DNN site is using.  Upon clicking the icon, a cookie is set.  When the page reloads, the skin in the cookie is used instead of the skin that is specified in the Page/Site Settings.

function sizeLayout(size){    setSkinCookie("DotNetNuke_Skin_Layout_Size",size, new Date("December 31, 2010 23:59:59"),"/");    location.reload();}
function setSkinCookie(name, value, expires, path, domain, secure) {  var curCookie = name + "=" + escape(value) +      ((expires) ? "; expires=" + expires.toGMTString() : "") +      ((path) ? "; path=" + path : "") +      ((domain) ? "; domain=" + domain : "") +      ((secure) ? "; secure" : "");  document.cookie = curCookie;}

I copied the code below from the DNN site into my site, but nothing (other than a page refresh) happens when clicking the images.

Feeling a bit dense here... I am not sure how setting this cookie value is supposed to work. How does DNN know which skin to use? If I have 10 skins with different names on my site, just providing a number (e.g., 0, 1, 2) does not seem to be enough info to indicate the correct skin. Is the skin supposed to have a specific name with a numeric value appended to the end?

Thanks in advance for your wisdom!

Code copied from DNN site:
<script language="Javascript">
function setSkinCookie(name, value, expires, path, domain, secure) {
  var curCookie = name + "=" + escape(value) +
      ((expires) ? "; expires=" + expires.toGMTString() : "") +
      ((path) ? "; path=" + path : "") +
      ((domain) ? "; domain=" + domain : "") +
      ((secure) ? "; secure" : "");
  document.cookie = curCookie;
}
</script>
<script language="Javascript">
function sizeText(size)
{
    setSkinCookie("DotNetNuke_Skin_Text_Size", size, new Date("December 31, 2010 23:59:59"),"/");
    location.reload();
}
</script>

<a href=" void(0)"><img src="/Portals/_default/Skins/DNN-Minimal/controls/images/icon_textsmall_on.gif" onClick="sizeText(0)" width="17" height="17" alt="Small text" border="0"></a>
<a href=" void(0)"><img src="/Portals/_default/Skins/DNN-Minimal/controls/images/icon_textmedium.gif" onClick="sizeText(1)" width="17" height="17" alt="Medium text" border="0"></a>
<a href=" void(0)"><img src="/Portals/_default/Skins/DNN-Minimal/controls/images/icon_textlarge.gif" onClick="sizeText(2)" width="17" height="17" alt="Large text" border="0"></a>