logo in header now stays white in light theme and reddish in dark theme

docs/stylesheets/extra.css

@@ -60,3 +60,8 @@
   color: var(--md-accent-fg-color-invert);
   text-decoration: underline dotted var(--md-accent-fg-color-invert);
 }
+
+/* Use reddish logo when in dark mode */
+[data-md-color-scheme="slate"] img[alt="logo"] {
+  content: url("../../img/schdesign_s_logo_red.svg");
+}
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