[web-animations][editorial] Use darkmode-aware colors.

Author: tabatkins

web-animations-1/Overview.bs

@@ -6,6 +6,11 @@
   background: #efe;
   border: green 1px dotted;
 }
+@media (prefers-color-scheme: dark) {
+  .informative-bg {
+    background: rgba(255 255 255 .05);
+  }
+}
 
 div.informative-bg *:last-child {
   margin-bottom: 0;
@@ -45,7 +50,7 @@ div.switch > dl > dt:before {
   font: bold 100% sans-serif;
   text-align: left;
   margin: 1.33em 0px;
-  color: #005A9C;
+  color: var(--heading-text, #005A9C);
 }
 
 .attributes::before { content: 'Attributes' }