freaCMS/app/views/colors.css

html {
    background-color: {{ @backgroundColor }};
    background-image: url(/{{ @backgroundImage }});
    background-position: bottom right;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size: 2000px 2000px;
}

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