How to enable and disable a bootstrapped design button/dropdown list

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C#

ASP.NET

HTML

bootstrap

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I have a ASP dropdown list which is designed with bootstrap. I have to enable another dropdown on an list item named "Others".

The Drop-down control which needs to be enabled is "ddlotheritem" on selectedindexchanged of "ddlitemtype" is written as

<div class="form-group">
     <label>Item Type
           <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ErrorMessage="*" ForeColor="Red" Font-Size="Large" Font-Bold="true"
            ControlToValidate="ddlitemtype" Display="Dynamic" InitialValue="Select" ValidationGroup="textboxes"></asp:RequiredFieldValidator>
     </label>
                            <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
                                <ContentTemplate>
                                    <asp:DropDownList ID="ddlitemtype" class="form-control" runat="server" ValidationGroup="textboxes"
                                        EnableViewState="true" AutoPostBack="true" ViewStateMode="Enabled"
                                        OnSelectedIndexChanged="ddlitemtype_SelectedIndexChanged">
                                    </asp:DropDownList>
                                </ContentTemplate>
                            </asp:UpdatePanel>
                        </div>



 <div class="form-group">
 <label>Others(Type)</label>
 <asp:DropDownList ID="ddlotheritem" class="form-control" runat="server" ValidationGroup="textboxes" disabled></asp:DropDownList>
 </div>

My problems is as follows.

1) As it is bootstrap design, if I am making Enabled="false" to "ddlotheritem" , the bootstrap designed is gone and a normal ASP dropdown without any design will come. When I add disabled to the tag as above code written, the design is working correctly as disabled text box. If I remove this 'disabled' tag and put Enable="false" in page load also, same thing happens i.e. without a design simple ASP dropdown which I don't want.

However, if this follows, I could not change the control to enabled on the backend code. The code I have written is

protected void ddlitemtype_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddlitemtype.SelectedItem.Text.ToUpper() == "OTHERS")
        {
            ddlotheritem.Attributes["enabled"] = "enabled";
            ddlotheritem.Items.Add("text"); // for testing adding a value
        }
        else
        {
            ddlotheritem.Attributes["disabled"] = "disabled";
        }
    }

How can I enable/disable the dropdown box "ddlotheritem" without any design change in Bootstrap on click of the Value "Others" of the dropdown "ddlitemtype" ?

To be noted that this page where this dropdown exists is a content page of a Master.

What I have tried:

As it is bootstrap design, if I am making Enabled="false" to "ddlotheritem" , the bootstrap designed is gone and a normal ASP dropdown without any design will come. When I add disabled to the tag as above code written, the design is working correctly as disabled text box. If I remove this 'disabled' tag and put Enable="false" in page load also, same thing happens i.e. without a design simple ASP dropdown which I don't want.