Powermail ext doesn't include css files needed for date picker - typo3

Question

TYPO3 7.6.15
Powermail 3.18.0
The date picker field does not open the popup calendar. I have narrowed down the cause being because the necessary CSS files are not being included for this feature. Is there an error in my setup? How do I have them included automatically?
This is what I have in my TS Setup:
plugin.tx_powermail {
view {
templateRootPaths >
templateRootPaths {
0 = {$plugin.tx_powermail.view.templateRootPath}
1 = fileadmin/template/mysite/powermail/Templates/
}
partialRootPaths >
partialRootPaths {
0 = {$plugin.tx_powermail.view.partialRootPath}
1 = fileadmin/template/mysite/powermail/Partials/
}
layoutRootPaths >
layoutRootPaths {
0 = {$plugin.tx_powermail.view.layoutRootPath}
1 = fileadmin/template/mysite/powermail/Layouts/
}
}
_LOCAL_LANG.default {
datepicker_format_date = d/m/Y
datepicker_format_time = H:i
datepicker_format_datetime = d/m/Y H:i
}
}
The only way I have found to make it work correctly is by manually adding it like this:
page.includeCSS {
file4 = EXT:powermail/Resources/Public/Css/Basic.css
}

Answer 1

in the Documentation ist a hint:
https://docs.typo3.org/typo3cms/extensions/powermail/ForAdministrators/Installation/Index.html#addbootstrapclassesandcsstopowermail
you should
add the static template Add classes and CSS based on bootstrap (powermail)
There the css is included

Answer 2

You need to tell powermail to load the JavaScript libraries by setting a constant in your TypoScript template:
plugin.tx_powermail.settings.javascript.addAdditionalJavaScript = 1
See https://docs.typo3.org/typo3cms/extensions/powermail/Faq/Index.html#datetimepickernotworking for more information.

Answer 3

Adding the static template file would result in a TypoScript like
plugin.tx_powermail.settings.BasicCss
=EXT:powermail/Resources/Public/Css/Basic.css
this should load everything.
The datepicker styles are originally located in https://github.com/einpraegsam/powermail/blob/develop/Resources/Private/Sass/_DatePicker.scss