How to add block to main container from custom theme?

I’ve created a custom theme: app/design/frontend/TestVendor/testTheme with registration.php and theme.xml files.

Inside the testTheme folder I’ve created another folder: TestFolder and inside this folder another 2 folders and files inside them:

1. TestFolder/layout/default.xml
2. TestFolder/templates/mainpage.phtml

This is how the structure looks like:

-TestVendor
—testTheme
——registration.php
——theme.xml
——TestFolder
——–templates
———–mainpage.phtml
———layout
———–default.xml

This is how default magento html skeleton looks like:

I want to add a new content inside the div element with the class name of “column main”. I searched and this html was defined in vendor/magento/module-theme/view/base/page_layout/empty.xml as the container tag with the name of “main”

So I just did 2 things:

1. added html tags with texsts to templates/mainpage.phtml.
2. configured the layout/default.xml file and applied the theme to the website

This is how my default.xml file looks like:

<?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">
    <referenceContainer name="main">
        <block class="MagentoFrameworkViewElementTemplate" name="slide" template="TestFolder::mainpage.phtml"/>
    </referenceContainer>
</page>

But mainpage.phtml content is not being rendered inside the “column main” div element. Am I doing something wrong? Any kind of help would be apreciated