1) Create a custom child theme that uses the ultimo theme as a parent:
app/code/design/frontend/YOURCOMPANY/YOURTHEME/theme.xml
<?xml version="1.0"?>
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
<title>YOURCOMPANY YOURTHEME</title>
<parent>Infortis/ultimo</parent>
</theme>
2) create app/code/design/frontend/YOURCOMPANY/YOURTHEME/registration.php
file
<?php
\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::THEME,
'frontend/YOURCOMPANY/YOURTHEME',
__DIR__
);
3) create app/design/frontend/YOURCOMPANY/YOURTHEME/Magento_Theme/layout/default_head_blocks.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<!-- favicons -->
(...)
<!-- Custom Metas -->
<meta name="msapplication-TileColor" content="#ffffff"/>
<meta name="msapplication-TileImage" content="icons/favicons/ms-icon-144x144.png"/>
<meta name="theme-color" content="#ffffff"/>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1.0, user-scalable=no"/>
<!-- ADD YOUR OTHER METAS HERE .... -->
</head>
</page>
4) Enable your custom theme in the admin area.
5) Read up for more info: https://devdocs.magento.com/guides/v2.2/frontend-dev-guide/themes/theme-inherit.html