If page_theme is an int, you need to supply it an int for your update to work:
protected void Button3_Click(object sender, EventArgs e)
{
con.open();
cmd = new SqlCommand("update marriage_configuration set page_theme = " + DropDownList1.SelectedItem.Value + " where u_e_id=(select R_id from users where Email_id=R_id)", con);
}
Quoting a value you concatenate into a sql string means it won't be interpreted as an integer, more likely a string type of some sort.
Also, don't ever concatenate SQL strings like that. You're just asking to get hacked by means of SQL injection. Use parameterized queries:
https://www.owasp.org/index.php/SQL_Injection[
^]