Go to app/design/frontend/Project/default/Magento_Theme/layout/default_head_blocks.xml
There you can define your custom js and css
Here is an example with multiples data
<?xml version="1.0"?>
<!--
/**
* Copyright © 2016 Magento. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<meta name="robots" content="INDEX,FOLLOW"/>
<css src="sprites/sprites.css"/>
<css src="glyphs/style.css"/>
<css src="slick/slick.css"/>
<css src="slick/slick-theme.css"/>
<css src="magnific_popup/magnific-popup.css"/>
<link src="js/custom.js"/>
</head>
</page>