Add Parameterized SCSS Mixins as a css til

css/parameterized-scss-mixins.md

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+# Parameterized SCSS Mixins
+
+A mixin can be made to be much more versatile by parameterizing it. If you
+need variations of a block of CSS, then move the parts that vary out into
+parameters to the mixin.
+
+```css
+@mixin navigation($background-color, $color, $link-color) {
+  nav {
+    display: flex;
+    justify-content: space-around;
+    background-color: $background-color;
+    color: $color;
+
+    ul {
+      list-style: none;
+
+      li a {
+        text-decoration: none;
+        color: $link-color;
+      }
+    }
+  }
+}
+
+div.base-nav {
+  @include navgation(#fff, #444, #222);
+}
+
+div.admin-nav {
+  @include navgation(#000, #fff, #ddd);
+}
+```
+
+The mixin can now easily be used to customize different segments of your
+app's styling.