Add CSS for external links.

in/css/all.sass

@@ -1,4 +1,15 @@
 @import "extra.scss"
+a[href^="http"] //external links
+	color: var(--extlink,lightblue)
+	::after
+		content: ""
+		width: 11px
+		height: 11px
+		background-image: url(var(--extimg, "/img/extlink.svg"))
+		background-position: center
+		background-repeat: no-repeat
+		background-size: contain
+		display: inline-block
 body
 	@include font("sans")
 .header

in/img/extlink.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<svg xmlns='http://www.w3.org/2000/svg' width='16' height='16' fill='currentColor' viewBox='0 0 16 16'><path fill-rule='evenodd' d='M8.636 3.5a.5.5 0 0 0-.5-.5H1.5A1.5 1.5 0 0 0 0 4.5v10A1.5 1.5 0 0 0 1.5 16h10a1.5 1.5 0 0 0 1.5-1.5V7.864a.5.5 0 0 0-1 0V14.5a.5.5 0 0 1-.5.5h-10a.5.5 0 0 1-.5-.5v-10a.5.5 0 0 1 .5-.5h6.636a.5.5 0 0 0 .5-.5z'/><path fill-rule='evenodd' d='M16 .5a.5.5 0 0 0-.5-.5h-5a.5.5 0 0 0 0 1h3.793L6.146 9.146a.5.5 0 1 0 .708.708L15 1.707V5.5a.5.5 0 0 0 1 0v-5z'/></svg>