neo theme files: auto-generate css vars

we do have boilerplate code inside our theme files:

$neoMap: map-merge($neoMap, (
    'boxlabel-background-color': #323232,
    'boxlabel-border'          : 0,
    'boxlabel-color'           : #ddd
));

@if $useCssVars == true {
    :root .neo-theme-dark { // .neo-box-label
        --boxlabel-background-color: #{neo(boxlabel-background-color)};
        --boxlabel-border          : #{neo(boxlabel-border)};
        --boxlabel-color           : #{neo(boxlabel-color)};
    }
}

we should check if the build and watch-themes scripts could auto-generate the part inside the if statement.