graphviz: render the generated SVGs pleasantly in dark mode

The generated SVGs now gain the `graphviz` class, which makes it easier
to customize their look via CSS. This is especially important to support
for dark mode, as there are other SVGs on the front page that should not
receive the same treatment.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

assets/sass/dark-mode.css

@@ -152,6 +152,10 @@
             #l10n-versions-dropdown footer a {
                 color: #6969dd;
             }
+
+            svg.graphviz {
+                filter: invert(60%);
+            }
         }
     }
 

layouts/_default/baseof.html

@@ -204,6 +204,7 @@ <h1 data-pagefind-meta="title">About{{ if (isset .Params "subtitle") }} - {{ .Pa
         },
       }
       const svg = vizInstance.renderSVGElement(x.innerText, options)
+      svg.classList.add("graphviz")
       x.parentNode.insertBefore(svg, x);
       x.style.display = 'none'
     });