tests(css): postcss popular plugins (#3959)

tests/css_postcss_plugins/postcss-mixins.css

@@ -0,0 +1,32 @@
+a {
+    @mixin $(theme)-colors;
+}
+
+@define-mixin icon $network, $color: blue {
+    .icon.is-$(network) {
+        color: $color;
+        @mixin-content;
+    }
+    .icon.is-$(network):hover {
+       color: white;
+       background: $color;
+   }
+}
+
+@mixin icon twitter {
+    background: url(twt.png);
+}
+@mixin icon youtube, red {
+    background: url(youtube.png);
+}
+
+.search {
+    @mixin icon search;
+}
+
+a {
+    color: black;
+    @mixin parent {
+        @mixin child;
+    }
+}