fix(styles): underline sidenote numerals

People seem to be confused about “disappearing” sidenotes on mobile.
This change introduces underlined sidenote numerals as an indicator that
you can click on the number.

Fixes #70.

css/tufte.css

@@ -483,7 +483,7 @@ p.subtitle {
 }
 
 .numeral {
-    font-family: et-book-roman-old-style;
+    font-family: "Libertinus Serif", sans-serif;
 }
 
 .danger {
@@ -666,7 +666,8 @@ img {
 
 .sidenote-number:after,
 .sidenote:before {
-    font-family: et-book-roman-old-style;
+    font-family: "Libertinus Serif", sans-serif;
+    font-variant-numeric: normal;
     position: relative;
     vertical-align: baseline;
 }
@@ -922,6 +923,10 @@ p.svg {
         display: none;
     }
 
+    .sidenote-number:after {
+        text-decoration: underline;
+    }
+
     .margin-toggle:checked+.sidenote,
     .margin-toggle:checked+.marginnote {
         display: block;