BEM_example.scss

$color--red: #FF0000;
$color--greed: #00FF00;
$color--blue: #0000FF;

$house--height: 20px;
$house__door--height: 6px;

@function house__window--color($color) {
    @return darker($color, 50%);
}

@mixin house__window($color) {
    border-color: house__window--color($color);
}

@mixin house($color) {
    &__left-window { @include house__window($color--red); }
    &__right-window { @include house__window($color--green); }

    &__door {
        height: house__door--height;
    }

    background-color: $color;
    height: $house--height;
}

.my-house {
    @include house($color--blue);
}