If Session["SessionTheme"] is not a string, as string will return null.
.ToString() will try to convert any other type to string by calling the object’s ToString() method. For most built-in types this will return the object converted to a string, but for custom types without a specific .ToString() method, it will return the name of the type of the object.
object o1 = "somestring";
object o2 = 1;
object o3 = new object();
object o4 = null;
string s = o1 as string; // returns "somestring"
string s = o1.ToString(); // returns "somestring"
string s = o2 as string; // returns null
string s = o2.ToString(); // returns "1"
string s = o3 as string; // returns null
string s = o3.ToString(); // returns "System.Object"
string s = o4 as string; // returns null
string s = o4.ToString(); // throws NullReferenceException
Another important thing to keep in mind is that if the object is null, calling .ToString() will throw an exception, but as string will simply return null.