Ejemplo Sass

test.scss

// variables

$color: rgba(0,0,0,.5);

// Mixins

@mixin large-text {
   font-family: Arial;
   font-weight: bold;
   font-size: 20px;
   color: #ff0000;
}

@mixin texto-negro {
    @include large-text;
    color: $color;
}

@mixin cajita {
    border-radius: 5px;
    border: 1px solid $color;
    box-shadow: 2px 2px 10px rgba(0,0,0,.5);
    padding: 20px;
}

// estilos generales

body {
    background-color: $color;
    @include texto-negro;
}
div {
    color: $color;
    span {
        border-color: $color;
    }
    &.azul {
        color: blue;
    }
    &:hover {
        text-decoration: underline;
    }
}

.page-title {
   @include large-text;
   padding: 4px;
   margin-top: 10px;
}

h2 {
    @include large-text;
}
.modulo-1 {
    @include cajita;
    background-color: yellow;
}
.modulo-2 {
    @include cajita;
    background-color: green;
    height: 100px;
    width: 100%;
}