man/custom-html: support dark mode

Use the browser's default style and opt in for dark mode.

Has the bonus of removing some
[deprecated attributes](https://developer.mozilla.org/docs/Web/HTML/Reference/Elements/body#deprecated_attributes).

man/custom-html.xsl

@@ -268,6 +268,10 @@
 <!-- add Index link at top of page -->
 <xsl:template name="user.header.content">
   <style>
+    :root {
+      color-scheme: light dark;
+    }
+
     a.headerlink {
       color: #c60f0f;
       font-size: 0.8em;
@@ -312,6 +316,13 @@
   <xsl:text>"</xsl:text>
 </xsl:template>
 
+<!-- copied from docbook to remove the static color declarations -->
+<xsl:template name="body.attributes">
+  <xsl:if test="starts-with($writing.mode, 'rl')">
+    <xsl:attribute name="dir">rtl</xsl:attribute>
+  </xsl:if>
+</xsl:template>
+
 <!-- Switch things to UTF-8, ISO-8859-1 is soo yesteryear -->
 <xsl:output method="html" encoding="UTF-8" indent="no"/>