WordPress post type icon

in functions.php file in the theme
SVG icon

$the_icon = file_get_contents( get_template_directory_uri( __FILE__ ) . '/icon_name.svg' );
"menu_icon" => 'data:image/svg+xml;base64,' . base64_encode( $the_icon ),

CSS

add_action('admin_head', 'my_custom_icon');
function my_custom_icon() {
echo  "<style>
.dashicons-my_custom_icon{
display: inline-block;
position: relative;
background-position: center;
background-repeat: no-repeat;
background-size: 100%;
margin: 0;
background-image: url('data:image/png;base64,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') ;
}
  </style>";
}
 
'menu_icon'           => 'dashicons-my_custom_icon',