From my conversation in the comments with you You're going to need to go to paste your CSS inside web/css and call it something like, lets say function.css. Save this inside that folder, next go to Vendor/Theme and if there's a Magento_Theme folder there go inside layout and click on default_head_blocks.xml. If not, you will need to create this file structure. (vendor/theme/magento_theme/layout/default_head_blocks.xml). Inside here you're going to want to insert <link rel="stylesheet" type="text/css" href="path/to/file/theme.css">
line just after
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
if you're making this file you then want to write
</head>
</page>
after it. Save, flush cache and reload the page and the file should be referenced in your head. I hope I've helped