docs: Add tweaks to the outline panel page (#18697)

Thought we could be extra clear here with the meaning of "singleton
buffers".

Release Notes:

- N/A

docs/theme/css/variables.css

@@ -26,6 +26,8 @@
   --border-light: hsl(220, 13%, 90%);
   --border-hover: hsl(220, 13%, 70%);
 
+  --media-bg: hsl(50, 25%, 92%);
+
   --sidebar-fg: hsl(0, 0%, 0%);
   --sidebar-non-existant: #aaaaaa;
   --sidebar-active: hsl(220, 93%, 42%);
@@ -98,6 +100,8 @@
   --border-light: hsl(220, 13%, 90%);
   --border-hover: hsl(220, 13%, 40%);
 
+  --media-bg: hsl(220, 13%, 8%);
+
   --sidebar-bg: hsl(220, 13%, 10%);
   --sidebar-fg: hsl(220, 14%, 71%);
   --sidebar-non-existant: #505254;