minimal-no-third-party/layouts/partials/css.html

<style>

    html body {
        font-family: '{{ .Site.Params.font }}', sans-serif;
        background-color: {{ .Site.Params.backgroundColor | default "white" }};
    }

    :root {
        --accent: {{ .Site.Params.accent | default "#2196F3" }};
        --border-width: {{ if .Site.Params.showBorder | default false }} 2px {{ else }} 0 {{ end }};
    }

</style>

<!-- main -->
<link rel="stylesheet" href="{{ "css/main.css" | absURL }}">