gistfile1.scss

$bleh: (
    color: red
);

$blah: (
    color: $blue-dark,
    font: 700 20px $font-header,
    line-height: 10px
);

$blah: () !default;

@mixin styles($styles, $defaults) {
    @each $name, $value in map-merge($defaults, $styles) {
        #{$name}: $value;
    }
}

dialog.mb-rename-group h1 {
    @include styles($blah, $bleh);
}