Improve <summary> CSS on sidebar

- Use a smaller font size for the toggle symbol. (Currently, it seems a little too large)
- Use the child combinator (`>`) to unify selectors.
- Use `margin-left` instead of whitespace within the `content` property.
- Use `::` instead of outdated `:` for the pseudo-element symbol.
  (See https://developer.mozilla.org/en-US/docs/Web/CSS/::before)

Author: ybiquitous

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -320,23 +320,26 @@ dl.note-list dt {
   background: url(../images/arrow_up.png) no-repeat right center;
 }
 
-.nav-section details summary {
+.nav-section details > summary {
   display: block;
 }
 
-.nav-section details summary::-webkit-details-marker {
+.nav-section details > summary::-webkit-details-marker {
   display: none;
 }
 
-.nav-section details summary:before {
+.nav-section details > summary::before {
   content: "";
 }
 
-.nav-section details summary:after {
-  content: "  \25B6"; /* BLACK RIGHT-POINTING TRIANGLE */
+.nav-section details > summary::after {
+  content: "\25B6"; /* BLACK RIGHT-POINTING TRIANGLE */
+  font-size: 0.8em;
+  margin-left: 0.4em;
 }
-.nav-section details[open] > summary:after {
-  content: "  \25BD"; /* WHITE DOWN-POINTING TRIANGLE */
+
+.nav-section details[open] > summary::after {
+  content: "\25BD"; /* WHITE DOWN-POINTING TRIANGLE */
 }
 
 /* @end */