Add support for aria-current="true"

Simply targeting [aria-current] is too broad so instead I’m specifically adding support for aria-current="true".

simple.css

@@ -289,7 +289,8 @@ header > nav a:visited {
 
 header > nav a:hover,
 header > nav a.current,
-header > nav a[aria-current] {
+header > nav a[aria-current="page"],
+header > nav a[aria-current="true"] {
   border-color: var(--accent);
   color: var(--accent);
   cursor: pointer;