[jashwant]

I added class="form-control" to input and btn class to submit button.

Hope this helps.

function cb_bp_directory_members_search_form() {
  $search_form_html = '<form action="" method="get" id="search-members-form">
    <label><input type="text" class="form-control" name="s" id="members_search" placeholder="'. esc_attr( $search_value ) .'" /></label>
    <input type="submit" id="members_search_submit" class="btn"  name="members_search_submit" value="' . __( 'Search', 'buddypress' ) . '" />
  </form>';
  return $search_form_html;
}
add_filter('bp_directory_members_search_form', 'cb_bp_directory_members_search_form');