Hi
this is because if the style code in your custom theme .
Go to /purity_iii/local/css/themes/essai/bootstrap.css
Find below code
.dropdown-menu {
position: absolute;
top: 100%;
left: 0;
z-index: 1000;
display: none;
float: left;
min-width: 160px;
padding: 5px 0;
margin: 2px 0 0;
list-style: none;
font-size: 14px;
text-align: left;
background-color: #444444;
border: 1px solid #444444;
border-radius: 200px;
-webkit-box-shadow: 0 6px 12px rgba(0,0,0,0.175);
box-shadow: 0 6px 12px rgba(0,0,0,0.175);
background-clip: padding-box;
}
replace it with
.dropdown-menu {
position: absolute;
top: 100%;
left: 0;
z-index: 1000;
display: none;
float: left;
min-width: 160px;
padding: 5px 0;
margin: 2px 0 0;
list-style: none;
font-size: 14px;
text-align: left;
background-color: #444444;
border: 1px solid #444444;
border-radius: 0px;
-webkit-box-shadow: 0 6px 12px rgba(0,0,0,0.175);
box-shadow: 0 6px 12px rgba(0,0,0,0.175);
background-clip: padding-box;
}