---
Name: myproject
---
SilverStripeCoreManifestModuleManifest:
  project: app
---
SilverstripeSiteConfigSiteConfig:
  extensions:
    - CustomSiteConfig
---
SilverStripeAdminLeftAndMain:
  extra_requirements_css:
    - public/resources/admin/css/custom.css

<?php

use SilverStripeFormsFieldList;
use SilverStripeFormsHTMLEditorHTMLEditorField;
use SilverStripeORMDataExtension;

class CustomSiteConfig extends DataExtension
{

    private static $db = [
        'FooterContent' => 'HTMLText'
    ];

    public function updateCMSFields(FieldList $fields)
    {
        $fields->addFieldToTab("Root.Main",
            new HTMLEditorField("FooterContent", "Footer Content")
        );
    }
}