In the theme folder, i define a css as:
.QuestionText
{
   margin-top:20px;
   text-align:center;
}

Here, i use it:
<asp:Label ID="question" runat="server" Height="96px" Text="Label"
Width="648px" BackColor="WhiteSmoke" BorderColor="CornflowerBlue"
BorderStyle="Solid" CssClass="QuestionText"></asp:Label>

but it takes no effect, why?

thanks.