Bring some responsive styles, subject to later improvements

src/assets/less/variables.less

@@ -1,3 +1,14 @@
 @primary-color: #0c9696;
 @secondary-color: #ffffff;
 @text-color: #333;
+
+@sm: 640px;
+// => @media (min-width: 640px) { ... }
+@md: 768px;
+// => @media (min-width: 768px) { ... }
+@lg: 1024px;
+// => @media (min-width: 1024px) { ... }
+@xl: 1280px;
+// => @media (min-width: 1280px) { ... }
+@2xl: 1536px;
+// => @media (min-width: 1536px) { ... }