flat-ui-select2.css

.select2-container .select2-choice {
    border: 2px solid #dce4ec;
    font-family: "Lato", sans-serif;
    font-size: 14px;
    text-indent: 1px;
    -webkit-border-radius: 6px;
       -moz-border-radius: 6px;
            border-radius: 6px;
    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;
}
.select2-container.select2-drop-above .select2-choice {
    border-bottom-color: #dce4ec;
    -webkit-border-radius:0 0 6px 6px;
       -moz-border-radius:0 0 6px 6px;
            border-radius:0 0 6px 6px;
}
.select2-drop {
    margin-top: -2px;
    border: 2px solid #dce4ec;
    border-top: 0;
    -webkit-border-radius:0 0 6px 6px;
       -moz-border-radius:0 0 6px 6px;
            border-radius:0 0 6px 6px;
    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;
}
.select2-drop.select2-drop-above {
    margin-top: 2px;
    border-top: 2px solid #dce4ec;
    border-bottom: 0;
    -webkit-border-radius: 6px 6px 0 0;
       -moz-border-radius: 6px 6px 0 0;
            border-radius: 6px 6px 0 0;
    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;
}
.select2-container .select2-choice div {
    border-left: 2px solid #dce4ec;
    -webkit-border-radius: 0 4px 4px 0;
       -moz-border-radius: 0 4px 4px 0;
            border-radius: 0 4px 4px 0;

    -webkit-background-clip: padding-box;
       -moz-background-clip: padding;
            background-clip: padding-box;
}
.select2-search input {
    height: 30px !important;
    border: 2px solid #dce4ec;
}
.select2-container-active .select2-choice,
.select2-container-active .select2-choices {
    border: 2px solid #dce4ec;
    outline: none;

    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;
}
.select2-dropdown-open .select2-choice {
    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;

    -webkit-border-bottom-left-radius: 0;
        -moz-border-radius-bottomleft: 0;
            border-bottom-left-radius: 0;

    -webkit-border-bottom-right-radius: 0;
        -moz-border-radius-bottomright: 0;
            border-bottom-right-radius: 0;
}
.select2-dropdown-open .select2-choice div {
    background: transparent;
    border-left: none;
    filter: none;
}
.select2-results .select2-highlighted {
    background: #3875d7;
    background: #1abc9c;
    color: #fff;
    -webkit-border-radius: 4px;
       -moz-border-radius: 4px;
            border-radius: 4px;
}
.select2-container-multi .select2-choices {
    height: auto !important;
    height: 1%;

    border: 2px solid #dce4ec;
}
.select2-container-multi.select2-container-active .select2-choices {
    border: 2px solid #dce4ec;
    -webkit-border-radius: 6px;
       -moz-border-radius: 6px;
            border-radius: 6px;

    -webkit-box-shadow: none;
       -moz-box-shadow: none;
            box-shadow: none;
}

Thanks for sharing!

I'm going to try and make it match the Flat UI dropkick select style if I can. I'll fork your work if I get chance!

I tried to make it little better...below is the link for it. Hope it help
https://github.com/techhysahil/Select2-Flat-UI/blob/master/Overide.css

If anybody wants demo Pen.
Demo : http://codepen.io/techhysahil/pen/bVJPLo