使用bootstrap 4类折叠时如何更改导航栏的背景颜色？

Question

我想在折叠时更改导航栏的背景色，否则使用引导程序4的类将背景色设为无。

   <nav class="navbar navbar-light justify-content-end bg-color">
                    <button class="navbar-toggler" type="button" data- 
    toggle="collapse" data-target="#thetarget" ><span class="navbar-toggler- 
    icon"></span>
                    </button>
                    <div class="row navbar-collapse collapse" id="thetarget" 
    style="text-align:center;">
                        <ul class="col-md-8 navbar-nav nav">
                            <li class="nav-item"><a href="#home" class="nav- 
    link">Home</a></li>
                            <li class="nav-item"><a href="#watchvideos" 
    class="nav-link">Watch Videos</a></li>


                            <li class="nav-item"><a href="#MoreTemplates2" 
    class="nav-link">More Templates 2</a></li>
                        </ul> 



                    </div>
                </nav>