Themes can be applied in 3 ways
1. Folder Level
2. Application Level
3. Page Level.
if u want to switch the themes you can apply based on the condition.
To apply a page theme progmatically
switch (Request.QueryString["theme"])
{
case "Blue":
Page.Theme = "BlueTheme";
break;
case "Pink":
Page.Theme = "PinkTheme";
break;
}
<b>To apply control skins programmatically</b>
ControlerID.SkinID = "CustomSkin";