Fix navbar text and background colors in dark mode

Drive the navbar text from the theme-reactive --hd-fg-* vars instead of
Bootstrap's baked-in dark color, so it flips light when the background
goes dark. Also fade the beginscroll gradient to --hd-bg-canvas rather
than a hardcoded white.

layouts/partials/nav.html

@@ -38,9 +38,9 @@
                     </a>
                     <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                         {{ range .Children }}
-                        <a href="{{ .URL }}" class="dropdown-item"
-                            >{{ .Name }}</a
-                        >
+                        <a href="{{ .URL }}" class="dropdown-item">
+                            {{ .Name }}
+                        </a>
                         {{ end }}
                     </div>
                     {{ else }}