style_Rmd.css: add contour for answers

www/style_Rmd.css

@@ -27,6 +27,19 @@ body {
   font-size: 22px;
 }
 
+details {
+  border: 4px;
+  border-style: solid;
+  border-color: #999999;
+  background-color: #E6E6E6;
+  padding-right: 1em;
+  padding-left: 1em;
+  text-align: left;
+  border-radius: 5px 4px 3px 2px;
+  font-weight: 300;
+  font-size: 22px;
+}
+
 @font-face {
   font-family: "Yanone Kaffeesatz";
   src: url("../www/YanoneKaffeesatz-Bold.ttf");