Feature icons: masked span painted by currentColor, not <img>

An image can't take the text color; a mask-image painted by
background-color can. Icons now follow --feature-accent when the
feature declares a color, and the surrounding text color otherwise.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

style/main.css

@@ -365,6 +365,13 @@ main.not-found {
   width: 1.3rem;
   height: 1.3rem;
   margin: 0.1rem 0.6rem 0.4rem 0;
+  background-color: var(--feature-accent, currentColor);
+  mask-repeat: no-repeat;
+  mask-size: contain;
+  mask-position: center;
+  -webkit-mask-repeat: no-repeat;
+  -webkit-mask-size: contain;
+  -webkit-mask-position: center;
 }
 
 .feature h3 {