Created a new test theme

themes/my-theme/assets/css/main.css

@@ -0,0 +1,22 @@
+body {
+  color: #222;
+  font-family: sans-serif;
+  line-height: 1.5;
+  margin: 1rem;
+  max-width: 768px;
+}
+
+header {
+  border-bottom: 1px solid #222;
+  margin-bottom: 1rem;
+}
+
+footer {
+  border-top: 1px solid #222;
+  margin-top: 1rem;
+}
+
+a {
+  color: #00e;
+  text-decoration: none;
+}