_mixins.scss

// Mixin for alternate theme styling
@mixin theme( $blogname ){
	body.theme-#{$blogname} & {
		@content;
	}
}

// Usage
.header {
	color: #000;
	background: #fff;
	@include theme('autumn') {
		color: #fff;
		background: #000;
	}
}

// Output
.header {
	color: #000;
	background: #fff;
}
body.theme-autumn .header {
	color: #fff;
	background: #000;
}