Magento 2.3.5 – Fix ‘Refused to Load the Stylesheet’ Error

I couldn't reproduce your error, but I got an other one.

Then I did some minor adjustments to your code and it worked.
First drop the InstallSchema.php file. It does nothing so it is useless.

Then make sure that your plugin file is in Amaj/CorePlugin/Model/Currency/Plugin.php and it looks like this:

<?php

namespace Amaj\CorePlugin\Model\Currency;

class Plugin
{
    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->logger = $logger;
    }

    /**
     * @param $subject
     * @param \Closure $proceed
     * @param $price
     * @param array $options
     * @return string
     */
    public function aroundFormatTxt(
        $subject,
        \Closure $proceed,
        $price,
        $options = []
    )
    {
        $this->logger->info('Amaj aroundFormatTxt');
        return '2';
    }

}

you don't need parent::__construct(); in the constructor because there is no parent class.
And I replaced $logger->info with $this->logger->info because there is no $logger variable.

It is recommendable use the plugin instead of preference for this.

https://devdocs.magento.com/guides/v2.2/extension-dev-guide/plugins.html

You must do the next.

Vendor/Namespace/etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="Magento\Sales\Controller\Adminhtml\Order\Index">
    <plugin name="vendor_namespace_plugin_list" type="Vendor\Namespace\Plugin\ControllerOrderIndexPlugin"/>
</type>

Vendor/Namespace/Plugin/ControllerOrderIndexPlugin.php

<?php

namespace Vendor\Namespace\Plugin;

use Magento\Framework\Message\ManagerInterface;
use Magento\Sales\Controller\Adminhtml\Order\Index;

/**
 * Class ControllerOrderIndexPlugin
 */
class ControllerOrderIndexPlugin
{

    /**
     * @var ManagerInterface
     */
    private $messageManager;

    /**
     * ControllerOrderIndexPlugin constructor.
     *
     * @param ManagerInterface $messageManager
     */
    public function __construct(ManagerInterface $messageManager)
    {
        $this->messageManager = $messageManager;
    }

    /**
     * @param Index $subject
     */
    public function beforeExecute(
        Index $subject
    ) {
        $this->messageManager->addSuccessMessage(__('Message from new admin controller.'));
    }
}