If you click the drop-down menu, the boot class added the "open" class so that you can overwrite it as follows:
.navbar-default .navbar-nav > .open > a,
.navbar-default .navbar-nav > .open > a:focus,
.navbar-default .navbar-nav > .open > a:hover {
color: #555;
background-color: #e7e7e7;
}
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