Magento 2 x Amasty override less_to_css.xml

I’m using my own compiler on my theme for less. I succesfully add the Amasty less files in my theme to override Amasty’s default theme. However, Amasty is using a custom compiler (cssOptions) to generate a CSS and include it in the header.

I tried it with several approaches.

• Override etc/less_to_css.xml

Original

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:amasty:module:Amasty_Base:etc/less_to_css.xsd">
    <module name="Amasty_CheckoutCore">
        <handle name="checkout_index_index" />
        <cssOptions fileName="amcheckout" path="css/source/mkcss" />
        <ifconfig>amasty_checkout/general/enabled</ifconfig>
    </module>
</config>

Edited

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:amasty:module:Amasty_Base:etc/less_to_css.xsd">
    <module name="Amasty_CheckoutCore">
        <handle name="checkout_index_index" />
        <ifconfig>amasty_checkout/general/enabled</ifconfig>
    </module>
</config>

But that doesn’t work (fairly logical I pressume). I tried it with copying over the file

app/design/Theme/base/Amasty_CheckoutCore/styles/mkcss/amcheckout.less

This is the file

ing css file. Path from vendor/magento/theme-frontend-blank
@import (reference) '../../../lib::css/source/lib/_lib.less'; // Library files
@import (reference) '../../../lib::css/source/lib/_responsive.less'; // Global responsive
@import '../_module.less';

Tried to remove it’s content..but it’s just not deleting the content or the reference to the files. The other .less files ARE compiling in my own compiler, so I know everything should be in the right place.