Display color picker in posts

Hello,
I want to display the colors choosen by the color picker I added to all my post.

I watched this video: https://www.youtube.com/watch?v=mjLsbRgQ2k4
I followed the steps the guy did, but it doesn't work for the post section.

I added this code in wp_header:

<div>
<style>
:root {
/<em>Begin First Varible Code</em>/
--background-color: <?php 
        $background_color = rwmb_meta( //Change the variable to your desired varible name
            'background_color_picker', //Color Picker ID
            ['object_type' => 'Post'], 
            'post' ); //Option Name from Settings Page 
        echo $background_color;
        ?>;
;
/<em>End First Varible Code</em>/
}
</style>
</div>

This are my fields I added to my posts:

<div>
 $meta_boxes[] = [
        'title'        => __( 'Extra Post Fields', 'your-text-domain' ),
        'id'           => 'extra-post-fields',
        'storage_type' => 'custom_table',
        'table'        => 'wp_mb_extra_post_fields',
        'tabs'         => [
            'tab_21kcpojc8ia' => [
                'label' => 'Color Tab ',
                'icon'  => 'dashicons-admin-appearance',
            ],
            'tab_kbam38nta9i' => [
                'label' => 'Video and featured image',
                'icon'  => 'dashicons-format-video',
            ],
            'tab_xlkmse8nuu'  => [
                'label' => 'Download Button 1',
                'icon'  => 'dashicons-upload',
            ],
            'tab_kv6sob7ra9q' => [
                'label' => 'Download Button 2',
                'icon'  => 'dashicons-upload',
            ],
        ],
        'fields'       => [
            [
                'name'          => __( 'Background color', 'your-text-domain' ),
                'id'            => $prefix . 'background_color_picker',
                'type'          => 'color',
                'alpha_channel' => true,
                'tab'           => 'tab_21kcpojc8ia',
            ],
            [
                'name' => __( 'Hide Featured image ', 'your-text-domain' ),
                'id'   => $prefix . 'text_a1fp2hdgzzt',
                'type' => 'text',
                'desc' => __( 'Type "yes" to hide | Leave empty to show', 'your-text-domain' ),
                'tab'  => 'tab_kbam38nta9i',
            ],
            [
                'name' => __( 'YouTube URL', 'your-text-domain' ),
                'id'   => $prefix . 'url_vdnpr9wzws',
                'type' => 'url',
                'tab'  => 'tab_kbam38nta9i',
            ],
            [
                'name'              => __( 'Video Embed', 'your-text-domain' ),
                'id'                => $prefix . 'wysiwyg_9s5x8ccyr3',
                'type'              => 'wysiwyg',
                'sanitize_callback' => 'none',
                'tab'               => 'tab_kbam38nta9i',
            ],
            [
                'name'    => __( 'Affiliate discosure', 'your-text-domain' ),
                'id'      => $prefix . 'select_rjsjhddfon',
                'type'    => 'select',
                'options' => [
                    'Commissions may be earned from the links above.​ If you purchase plugins with these links, I will receive a small commission at no additional cost to you. Thank you!' => __( 'Commissions may be earned from the links above.​ If you purchase plugins with these links, I will receive a small commission at no additional cost to you. Thank you!', 'your-text-domain' ),
                    'Not sponsored or affiliated, just sharing to help out the music production community.'                                                                                   => __( 'Not sponsored or affiliated, just sharing to help out the music production community.', 'your-text-domain' ),
                ],
                'tab'     => 'tab_xlkmse8nuu',
            ],
            [
                'name' => __( 'Download title', 'your-text-domain' ),
                'id'   => $prefix . 'text_fop15u4ojpf',
                'type' => 'text',
                'tab'  => 'tab_xlkmse8nuu',
            ],
            [
                'name' => __( 'Button text 1', 'your-text-domain' ),
                'id'   => $prefix . 'text_40hi0nvtvmw',
                'type' => 'text',
                'tab'  => 'tab_xlkmse8nuu',
            ],
            [
                'name' => __( 'Download Link 1', 'your-text-domain' ),
                'id'   => $prefix . 'url_f0t8ukl11vj',
                'type' => 'url',
                'tab'  => 'tab_xlkmse8nuu',
            ],
            [
                'name' => __( 'Button text 2', 'your-text-domain' ),
                'id'   => $prefix . 'text_ui1dobxqbif',
                'type' => 'text',
                'tab'  => 'tab_kv6sob7ra9q',
            ],
            [
                'name' => __( 'Download Link 2', 'your-text-domain' ),
                'id'   => $prefix . 'url_n93zhbdfdj',
                'type' => 'url',
                'tab'  => 'tab_kv6sob7ra9q',
            ],
        ],
    ];
</div>
<style>

Here is how if shows up in my website:

<div>

:root {
/<em>Begin First Varible Code</em>/
--background-color: ;
;
/<em>End First Varible Code</em>/
}
</style>
</div>

Can you please help me?
Thanks