Getting color form registry into CSS/style (Plone Classic)

Plone Support Add-on Development
1

IN manifest.cfg I can (probably) get the color like this:

csscolor = python:portal_state.portal().portal_registry['My.Addon.interfaces.IMyAddonSettings.color1']

How can I get this into my theme (withouth using themefragments as I have done before):

So basically, I just want to add:

<style>.someclass {background-color: $csscolor; } </style>

PS: I just need one color

2

I have done this registering a viewlet for plone.app.layout.viewlets.interfaces.IHtmlHead manager

ZCML:

    <browser:viewlet
        for="*"
        name="my.addon.htmlhead.custom_css"
        class=".custom_css.CustomCssViewlet"
        template="custom_css.pt"
        manager="plone.app.layout.viewlets.interfaces.IHtmlHead"
        permission="zope2.View"
        layer="...IMyLayer"
        />

PYTHON:

class CustomCssViewlet(ViewletBase):
    """ CustomCssViewlet
    """

    def update(self):
        super().update()
        self.color = api.portal.get_registry_record('prefix.color')

TEMPLATE:

<style tal:condition="view/color">
    :root {
        --my-variable: ${view/colour};
    }
</style>