freaCMS/app/views/colors.css

body {
    background-color: {{ @backgroundColor }};
    <check if="{{ isset(@backgroundImage) }}">
    background-image: url(/{{ @backgroundImage }});
    </check>
}

<repeat group="{{ @customVars }}" key="{{ @ikey }}" value="{{ @ivalue }}">
:root {
    --{{ @ikey }}: {{ $@ikey }};
}
</repeat>