[doc] Make png images readable in dark mode (#3935)

docs/modules/ROOT/pages/advancedFeatures/advanced-mutualizing-operator-usage.adoc

@@ -30,7 +30,7 @@ Flux.fromIterable(Arrays.asList("blue", "green", "orange", "purple"))
 
 The following image shows how the `transform` operator encapsulates flows:
 
-image::gs-transform.png[Transform Operator : encapsulate flows]
+image::gs-transform.png[Transform Operator : encapsulate flows, role="invert-dark"]
 
 The preceding example produces the following output:
 
@@ -77,7 +77,7 @@ composedFlux.subscribe(d -> System.out.println("Subscriber 2 to Composed MapAndF
 
 The following image shows how the `transformDeferred` operator works with per-subscriber transformations:
 
-image::gs-compose.png[Compose Operator : Per Subscriber transformation]
+image::gs-compose.png[Compose Operator : Per Subscriber transformation, role="invert-dark"]
 
 The preceding example produces the following output:
 

docs/modules/ROOT/pages/advancedFeatures/reactor-hotCold.adoc

@@ -61,7 +61,7 @@ Subscriber 2: PURPLE
 
 The following image shows the replay behavior:
 
-image::gs-cold.png[Replaying behavior]
+image::gs-cold.png[Replaying behavior, role="invert-dark"]
 
 Both subscribers catch all four colors, because each subscriber causes the
 process defined by the operators on the `Flux` to run.
@@ -105,7 +105,7 @@ Subscriber 2 to Hot Source: PURPLE
 
 The following image shows how a subscription is broadcast:
 
-image::gs-hot.png[Broadcasting a subscription]
+image::gs-hot.png[Broadcasting a subscription, role="invert-dark"]
 
 Subscriber 1 catches all four colors. Subscriber 2, having been created after the first
 two colors were produced, catches only the last two colors. This difference accounts for