styling with bootstrap 4

I am using bootstrap 4 in web site. how to apply bootstrap dropdown style to asp net controls. for example, asp:dropdown.

<asp:dropdown id="ddl" runat="server" /> .

also, when we use  bootstrap classes to create form and inputs , like below. how to add  vertical space between the inputs ?

<div class="form-control">
   <div class="row">
       <div class="col-6>
            <div class="form-group">
                   <asp:textbox id="txt" runat="server">
              ,<div class="form-group">
                     <asp:dropdown .....
mikha eAsked:
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Julian HansenCommented:
Bootstrap 4 has new classes for padding and spacing see here

So to add spacing below your inputs you could do
<div class="form-group mb-1 mt-1"> ...

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This will add a top (t) and bottom (b) margin (m) to the element. 1 is 25% of the spacer - which you can define the in SASS spacer map if you don't like the options (1,2,3) provided
Change N (mb-N) to value that matches your requirement in terms of spacing.

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