set styles as below
<asp:button id="Button5" runat="server" text="Herbal-Internal" class="stylebutton" causesvalidation="false" onclick="btnherbal_click" xmlns:asp="#unknown" />
your css:
.stylebutton {
cursor: pointer;
font-family: Verdana;
font-weight: normal;
margin-top: 28px;
margin-left: 0px;
border-style: none;
color: #666666;
background-color: white;
}
.stylebutton:hover {
cursor: pointer;
font-family: Verdana;
font-weight: normal;
margin-top: 28px;
margin-left: 0px;
border-style: none;
color: white;
background-color: Red;
}