use octicons for expander triangles in nav (PR #68)

Author: mojavelinux

src/css/nav.css

@@ -161,19 +161,19 @@
 }
 
 .nav-item-toggle {
-  background: transparent url(../img/caret.svg) no-repeat center / 50%;
+  background: transparent url(../img/octicons-16.svg#view-triangle-right) no-repeat center;
+  opacity: 0.25;
   border: none;
   outline: none;
   line-height: inherit;
   position: absolute;
   height: calc(var(--nav-line-height) * 1em);
   width: calc(var(--nav-line-height) * 1em);
-  margin-top: -0.05em;
-  margin-left: calc(var(--nav-line-height) * -1em);
+  margin-top: -0.05em 0 0 calc(var(--nav-line-height) * -1em);
 }
 
 .nav-item.is-active > .nav-item-toggle {
-  transform: rotate(90deg);
+  background-image: url(../img/octicons-16.svg#view-triangle-down);
 }
 
 .is-current-page > .nav-link,