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Solved for Joomla 4! I found this tracker entry which lead me to update the php at the beginning of admin.css.php:

<?php
define('_JEXEC', 1);
define('JPATH_BASE', __DIR__ . '/../../..');

require_once JPATH_BASE . '/includes/defines.php';
require_once JPATH_BASE . '/includes/framework.php';

// Boot the DI container.
$container = \Joomla\CMS\Factory::getContainer();
// UPDATED alias the session service key to the web session service.
$container->alias('session.web', 'session.web.site')
    ->alias('session', 'session.web.site')
    ->alias('JSession', 'session.web.site')
    ->alias(\Joomla\CMS\Session\Session::class, 'session.web.site')
    ->alias(\Joomla\Session\Session::class, 'session.web.site')
    ->alias(\Joomla\Session\SessionInterface::class, 'session.web.site');

$app            = $container->get(\Joomla\CMS\Application\SiteApplication::class);
$myTemplate = $app->getTemplate(true);
$params         = $myTemplate->params;

header("Content-Type:text/css; charset=UTF-8");
header("Cache-Control:must-revalidate");
?>

Atum (the default admin template for Joomla 4) makes targeting the button backgrounds a little tricky. In order for backgrounds to work you must add the class btn btn-outline-secondary to your field options

<field name="backgroundColor" label="Background Color" type="radio" >
        <option class="btn btn-outline-secondary bg01" value="color01">Color 01</option>
        <option class="btn btn-outline-secondary bg02" value="color02">Color 02</option>
        <option class="btn btn-outline-secondary bg03" value="color03">Color 03</option>
        etc
</field>

And in the admin.css.php, use the following for each button:

.bg01 {text-shadow: 0px 1px #000000; border-color: #495057; font-weight: bold; color:<?php echo $params->get('color01'); ?>; }
.btn-check:checked + .btn-outline-secondary.bg01 {text-shadow: 0px 1px #000000; color:#fff; background-color:<?php echo $params->get('color01'); ?>; }

This will create two color states for when the button is selected, or not selected. Here is the result:

UPDATE: In order for this method to work, the template that is using

<![CDATA[<link rel="stylesheet" type="text/css" href="../media/vendor/mycustomfolder/admin.css.php" />]]>

must be set as the default template otherwise the colors will not be displayed.