Refactor and split SCSS files, add mixins

_sass/mixins.scss

@@ -0,0 +1,23 @@
+@mixin for-size($size) {
+    @if $size == phone-only {
+        @media (max-width: 599px) {
+            @content;
+        }
+    } @else if $size == tablet-portrait-up {
+        @media (min-width: 600px) {
+            @content;
+        }
+    } @else if $size == tablet-landscape-up {
+        @media (min-width: 900px) {
+            @content;
+        }
+    } @else if $size == desktop-up {
+        @media (min-width: 1200px) {
+            @content;
+        }
+    } @else if $size == big-desktop-up {
+        @media (min-width: 1800px) {
+            @content;
+        }
+    }
+}