<style>
 
       .CssStyle1   
       { 
           font: 12pt verdana; 
           font-weight:700;
           color:orange;
       }
 
       .CssStyle2
       { 
           font: 15pt times; 
           font-weight:250;
           color:blue;
       }
 
   </style>
 
   <script language="C#" runat="server">
 
       void Button1_Click(Object sender, EventArgs e) {
           HyperLink1.CssClass=((HyperLink1.CssClass=="CssStyle1")?"CssStyle2":"CssStyle1");
       }
 
   </script>

------解决方案--------------------
<style>
  
      .CssStyle1  
      {  
          font: 12pt verdana;  
          font-weight:700;
          color:orange;
      }
  
      .CssStyle2
      {  
          font: 15pt times;  
          font-weight:250;
          color:blue;
      }
  
  </style>
  
  <script language="C#" runat="server">
  
      void Button1_Click(Object sender, EventArgs e) {
          HyperLink1.CssClass=((HyperLink1.CssClass=="CssStyle1")?"CssStyle2":"CssStyle1");
      }
  
  </script>

利用css控制