Skin talk:Vector/Customize

What is the best way to make a working copy of the current version of Vector that can then be adjusted?

I wanted to try something similar to DarkVector (a skin based on Vector with just some cosmetic adjustments), but when I duplicate the current 1.37 Vector and change out the skin name in the code and file names, it produces internal errors like this:

Class 'AltVector\SkinVersionLookup' not found.
Backtrace: from /var/www/my.wiki/public_html/mediawiki-1.37.2/skins/AltVector/includes/ServiceWiring.php(55)
...

Thanks for any thoughts/suggestions on how best to approach this!

If i inser this code. The section above (sidebar section) duplicates.

  case 'SEARCH':
    /* custom search engines begins */
    //$html = file_get_contents('custom-search.htm', FILE_USE_INCLUDE_PATH);
    $html = '<span>Example</span>';
    $portal['html-items'] .= $html;
    $props[] = $portal;
    /* custom search engines ends */
    break;