셀렉트 박스의 색상수정은 어디서 하면 될까요?

<!-- 게시판 검색 시작 { -->
    <fieldset id="bo_sch">
        <legend>게시물 검색</legend>

        <form name="fsearch" method="get">
        <input type="hidden" name="bo_table" value="<?php echo $bo_table ?>">
        <input type="hidden" name="sca" value="<?php echo $sca ?>">
        <input type="hidden" name="sop" value="and">
        <label for="sfl" class="sound_only">검색대상</label>
        <select name="sfl" id="sfl">
            <option value="wr_subject"<?php echo get_selected($sfl, 'wr_subject', true); ?>>제목</option>
            <option value="wr_content"<?php echo get_selected($sfl, 'wr_content'); ?>>내용</option>
            <option value="wr_subject||wr_content"<?php echo get_selected($sfl, 'wr_subject||wr_content'); ?>>제목+내용</option>
            <option value="mb_id,1"<?php echo get_selected($sfl, 'mb_id,1'); ?>>회원아이디</option>
            <option value="mb_id,0"<?php echo get_selected($sfl, 'mb_id,0'); ?>>회원아이디(코)</option>
            <option value="wr_name,1"<?php echo get_selected($sfl, 'wr_name,1'); ?>>글쓴이</option>
            <option value="wr_name,0"<?php echo get_selected($sfl, 'wr_name,0'); ?>>글쓴이(코)</option>
        </select>
        <label for="stx" class="sound_only">검색어<strong class="sound_only"> 필수</strong></label>
        <input type="text" name="stx" value="<?php echo stripslashes($stx) ?>" required id="stx" class="sch_input" size="25" maxlength="20" placeholder="검색어를 입력해주세요">
        <button type="submit" value="검색" class="sch_btn"><i class="fa fa-search" aria-hidden="true"></i><span class="sound_only">검색</span></button>
        </form>
    </fieldset>
    <!-- } 게시판 검색 끝 -->