Adding icon to each category in wordpress - php

Question

I tried to add icons to left of the category names with the following codes;
/** Add Custom Field To Category Form */
add_action( 'category_add_form_fields', 'category_form_custom_field_add', 10 );
add_action( 'category_edit_form_fields', 'category_form_custom_field_edit', 10, 2 );
function category_form_custom_field_add( $taxonomy ) {
?>
<div class="form-field">
<label for="category_custom_order">Simge Kodu</label>
<input name="category_custom_order" id="category_custom_order" type="text" value="" size="40" aria-required="true" />
<p class="description">Dökümanda bulunan simgelerden birini seçip kodunu buraya girin.</p>
</div>
<?php
}
function category_form_custom_field_edit( $tag, $taxonomy ) {
$option_name = 'category_custom_order_' . $tag->term_id;
$category_custom_order = get_option( $option_name );
?>
<tr class="form-field">
<th scope="row" valign="top"><label for="category_custom_order">Simge Kodu</label></th>
<td>
<input type="text" name="category_custom_order" id="category_custom_order" value="<?php echo esc_attr( $category_custom_order ) ? esc_attr( $category_custom_order ) : ''; ?>" size="40" aria-required="true" />
<p class="description">Dökümanda bulunan simgelerden birini seçip kodunu buraya girin.</p>
</td>
</tr>
<?php
}
/** Save Custom Field Of Category Form */
add_action( 'created_category', 'category_form_custom_field_save', 10, 2 );
add_action( 'edited_category', 'category_form_custom_field_save', 10, 2 );
function category_form_custom_field_save( $term_id, $tt_id ) {
if ( isset( $_POST['category_custom_order'] ) ) {
$option_name = 'category_custom_order_' . $term_id;
update_option( $option_name, $_POST['category_custom_order'] );
}
}
?>
After that, i added the following codes to the theme. I want to use get_option function to get icons which belongs to each categories. However, i cannot get category id, so it cannot work. How can i get category id's? I mean, in the code, get_option('category_custom_order_{menu ID}' ), {menu ID} should return the each category id. Can you please help, thanks.
<?php
$defaults = array(
'theme_location' => 'menu',
'menu' => 'Üst Menü',
'container' => '',
'container_class' => '',
'container_id' => '',
'menu_class' => 'menu',
'menu_id' => '',
'echo' => true,
'fallback_cb' => 'wp_page_menu',
'before' => '',
'after' => '',
'link_before' => '<i class="fa '.get_option('category_custom_order_{menu ID}' ).' fa-fw"></i> ',
'link_after' => '',
'items_wrap' => '%3$s',
'depth' => 0,
'walker' => ''
);
wp_nav_menu( $defaults );
?>