dark mode: dim images

As per the guidance provided in
https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/#aa-dark-mode-images
wht this commit we tone down the images in dark mode, applying
techniques to filter background images as described in
https://css-tricks.com/apply-a-filter-to-a-background-image/.

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

Author: dscho

assets/sass/dark-mode.css

@@ -22,6 +22,38 @@
         $callout-color:           #000000;
         $black-3:                 #cccccc;
         $main-bg:                 #000000;
+
+        body#{$class}::before {
+            background-image: url("/images/bg/body.jpg");
+            position: absolute;
+            content: "";
+            width: 100%;
+            height: 200%;
+            filter: brightness(.4) contrast(1.6);
+            z-index: -1;
+        }
+        body#{$class} {
+            background: transparent #000000;
+            div#masthead {
+                background: transparent;
+            }
+            div#masthead::before {
+                content: "";
+                width: 100%;
+                height: 295px;
+                background: url("/images/bg/isometric-grid.png") top right repeat;
+                position: absolute;
+                filter: brightness(.4) contrast(1.6);
+                background-size: 35px 21px;
+                z-index: -1;
+            }
+            div.monitor {
+                filter: brightness(.7) contrast(1.2);
+            }
+            img {
+                filter: brightness(.6) contrast(1.2);
+            }
+        }
     }
 
     body#{$class} {