dark-mode: add an alternative Git logo

This logo differs only in the colors (and in the format: it's an SVG),
designed to work better with dark mode.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

assets/sass/dark-mode.css

@@ -82,10 +82,15 @@
                 }
 
                 ol#about-nav li a {
-                    filter: brightness(0.9);
+                    filter: brightness(0.8);
                 }
             }
 
+            img[alt="Git"] {
+                filter: unset;
+                content: url(./images/logo.dark-mode.svg);
+            }
+
             a.subtle-button {
                 background-image: linear-gradient(#555555, #777777);
                 border-top: solid 1px #333333;