Select2 styles for Flat UI

select2-flat.css

/*
 * Select2 v3.4.6 styles customization for Flat UI
 */
/*----------------------------------------------- Main select element ------------------------------------------------*/
.select2-container .select2-choice {
    height: 41px; /* Jobsy form controls have 37px total height */
    border: 2px solid #bdc3c7;
    border-radius: 6px;
    outline: none;
    font: 15px/38px "Lato", Liberation Sans, Arial, sans-serif;
    color: #34495e;

    /* important - to keep height always as constant */
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;

    /* reset gradient */
    background-image: none;
    filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);

    -webkit-transition: border-color .25s;
    -moz-transition: border-color .25s;
    -ms-transition: border-color .25s;
    -o-transition: border-color .25s;
    transition: border-color .25s;
}

/* active state */
.select2-container-active .select2-choice,
.select2-container-active .select2-choices {
    border-color: #1abc9c;

    /* reset shadow */
    -webkit-box-shadow: none;
    box-shadow: none;
}

/* container state, when dropdown open */
.select2-dropdown-open .select2-choice {
    border-bottom: none;
    border-radius: 6px 6px 0 0;
    padding-bottom: 2px;
    background-color: #fff;

    /* reset shadow */
    -webkit-box-shadow: none;
    box-shadow: none;

    /* reset gradient */
    background-image: none;
    filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
}

/* container state, when dropup open */
.select2-dropdown-open.select2-drop-above .select2-choice,
.select2-dropdown-open.select2-drop-above .select2-choices {
    border: 2px solid #1abc9c;
    border-radius: 0 0 6px 6px;
    padding-bottom: 0;
    border-top: none;
    padding-top: 2px;
    background-image: none;
    filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
}

/* disabled state */
.select2-container.select2-container-disabled .select2-choice {
    background-color: #f4f6f6;
    border: 2px solid #d5dbdb;
    color: #d5dbdb;
    cursor: default;
}

/*-------------------------------------- X-icon (which clears selected result) ---------------------------------------*/
.select2-container .select2-choice abbr {
    top: 12px;
}

.select2-container-active.select2-drop-above .select2-choice abbr {
    top: 14px;
}

/*---------------------------------------------------- Down-arrow ----------------------------------------------------*/
.select2-container .select2-choice .select2-arrow {
    width: 22px;
    height: 27px;
    top: 5px;
    border: none;
    background: #fff;

    /* reset gradient */
    background-image: none;
    filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
}

.select2-dropdown-open.select2-drop-above .select2-choice .select2-arrow {
    top: 7px;
}

/*----------------------------------------------------- Dropdown -----------------------------------------------------*/
/* dropdown with options */
.select2-drop {
    -webkit-transition: border-color .25s;
    -moz-transition: border-color .25s;
    -ms-transition: border-color .25s;
    -o-transition: border-color .25s;
    transition: border-color .25s;

    /* reset shadow */
    -webkit-box-shadow: none;
    box-shadow: none;
}

/* dropup (case, when there is not enough space below the field) */
.select2-drop.select2-drop-above {
    /* reset shadow */
    -webkit-box-shadow: none;
    box-shadow: none;
}

/* active dropdown */
.select2-drop-active {
    border: 2px solid #1abc9c;
    border-top: none;
    border-radius: 0 0 6px 6px;
}

/* active dropup */
.select2-drop.select2-drop-above.select2-drop-active {
    border-top: 2px solid #1abc9c;
    border-radius: 6px 6px 0 0;
}

.select2-results .select2-result {
    font: 15px/38px "Lato", Liberation Sans, Arial, sans-serif;
    color: #34495e;
}

.select2-results .select2-highlighted {
    background-color: #1abc9c;
    color: #fff;
}

/* help-text line */
.select2-results .select2-no-results,
.select2-results .select2-searching,
.select2-results .select2-selection-limit {
    background: #fff;
    font: 15px/38px "Lato", Liberation Sans, Arial, sans-serif;
    color: #34495e;
}

/* "loading more results" message */
.select2-more-results.select2-active {
    background: #f4f4f4 url('select2-spinner.gif') no-repeat 100%;
    padding: 4px 7px;
}

/* input in dropdown */
.select2-search input {
    background: transparent;
    font: 15px/38px "Lato", Liberation Sans, Arial, sans-serif;
    color: #34495e;
}

/*----------------------------------------------- Retina displays fix ------------------------------------------------*/
@media only screen and (-webkit-min-device-pixel-ratio: 1.5), only screen and (min-resolution: 144dpi) {
    .select2-container .select2-choice abbr {
        margin-top: 1px;
    }

    .select2-container .select2-choice .select2-arrow {
        margin-top: 1px;
    }
}

Thanks for sharing!

I have edit this CSS to make it better.
https://github.com/techhysahil/Select2-Flat-UI
Demo : http://codepen.io/techhysahil/pen/qbrqWG