Use CSS filter for dark mode chart background, leave logos untouched

Apply invert filter to entire chart for dark mode background, but
don't re-invert logos - they will be inverted along with the chart
which should make them visible on the dark background.

content.py

@@ -1013,4 +1013,14 @@ h3 .header-link-icon {
     }
 }
 
+/* ====== Dark Mode Plotly Charts ====== */
+/* Invert chart background for dark mode, but leave logos untouched */
+.dark .js-plotly-plot {
+    filter: invert(0.88) hue-rotate(180deg);
+}
+
+/* Re-invert the modebar so icons display correctly */
+.dark .js-plotly-plot .modebar {
+    filter: invert(1) hue-rotate(180deg);
+}
 """