Add focus ring around Button component

components/Button.module.css

@@ -17,6 +17,16 @@
   cursor: pointer;
 }
 
+.button:focus,
+.link:focus {
+  box-shadow: 0 0 0 calc(4rem / 16) var(--secondary-accent);
+}
+
+.small.button:focus,
+.small.link:focus {
+  box-shadow: 0 0 0 calc(3rem / 16) var(--secondary-accent);
+}
+
 .link {
   text-decoration: none;
 }