You have to build a quick extension of yours which can reenable the field. You need to create folders and a file like following:
your_ext/Configuration/TCA/Overrides/tt_content.php
the contents of that file are:
use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
ExtensionManagementUtility::addTCAcolumns('tt_content',[
'header_position' => [
'exclude' => 1,
'label' => 'LLL:EXT:your_ext/Resources/Private/Language/locallang_db.xlf:tt_content.header_position',
'config' => [
'type' => 'select',
'renderType' => 'selectSingle',
'items' => [
['LLL:EXT:your_ext/Resources/Private/Language/locallang_db.xlf:tt_content.header_position.left', 'left'],
['LLL:EXT:your_ext/Resources/Private/Language/locallang_db.xlf:tt_content.header_position.right', 'right'],
['LLL:EXT:your_ext/Resources/Private/Language/locallang_db.xlf:tt_content.header_position.center', 'center']
]
]
]
]);
ExtensionManagementUtility::addFieldsToPalette('tt_content', 'header', '--linebreak--,header_position', 'after:header_layout');
ExtensionManagementUtility::addFieldsToPalette('tt_content', 'headers', '--linebreak--,header_position', 'after:header_layout');
Now the field should be back in the backend, because you've added it to the TCA via addTCAColumns to the tt_content configuration and added it via addFieldsToPalette to the header and headers palettes of tt_content, which are used by the types textmedia and header.
You can find out more about this by using the Configuration module in the TYPO3 backend. You can see it, when you are logged in as admin. Also a good place to look and learn about the TCA is the TCA reference: https://docs.typo3.org/typo3cms/TCAReference/
Now you need to alter the fluid_styled_content templates. You need to create template overrides for the header partial of fluid_styled_content.
First create a folder: your_ext/Configuration/TypoScript and add a setup.txt and a constants.txt file.
In setup.txt add the following lines:
lib.fluidContent{
templateRootPaths{
10 = {$plugin.your_ext.view.fluid_styled_content.templateRootPath}
}
partialRootPaths{
10 = {$plugin.your_ext.view.fluid_styled_content.partialRootPath}
}
layoutRootPaths{
10 = {$plugin.your_ext.view.fluid_styled_content.layoutRootPath}
}
}
In constants.txt do:
plugin.your_ext{
view{
fluid_styled_content{
templateRootPath = EXT:your_ext/Resources/Private/FluidStyledContent/Templates/
partialRootPath = EXT:your_ext/Resources/Private/FluidStyledContent/Partials/
layoutRootPath = EXT:your_ext/Resources/Private/FluidStyledContent/Layouts/
}
}
}
To enable your TypoScript, you need to add a ext_tables.php in your your_ext folder and give it the following one-liner:
TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addStaticFile($_EXTKEY,'Configuration/TypoScript', 'Your Ext Template');
You need to include your static TypoScript to your page via the Template module to enable the change to fluid_styled_content
Now you can copy the templates you need from
typo3/sysext/fluid_styled_content/Resources/Private/Templates
typo3/sysext/fluid_styled_content/Resources/Private/Partials
typo3/sysext/fluid_styled_content/Resources/Private/Layouts
into your extensions folders you need to create:
your_ext/Resources/Private/FluidStyledContent/Templates
your_ext/Resources/Private/FluidStyledContent/Partials
your_ext/Resources/Private/FluidStyledContent/Layouts
Now you can alter the templates. For your header_position field, you probably just need to copy
typo3/sysext/fluid_styled_content/Resources/Private/Partials/Heaeder.html
to
your_ext/Resources/Private/FluidStyledContent/Partials/Header.html
and add your selected value as {data.header_position} to a div class and style that.
Keep in mind you do not need to copy all of the templates, because with the TypoScript you just defined another location for fluid to search for templates and take them, if they are available. If not, fluid will walk back the chain and take the templates that are defined at position 9 and lower. You can look into the TypoScript Object Browser by the Template module and look into the TypoScript variable lib.FluidContent to see, if your TypoScript include has worked.
Hope this helped a bit ;)