Added a light variant of the arrow image.

nightMode.css loads this instead of the regular dark arrow.
ResizableRaceCanvas doesn't cache the race boundary background, as it was unneccessary and stopped the canvas from being transparent.
#story[1093]

racevisionGame/src/main/resources/css/dayMode.css

@@ -50,4 +50,6 @@
 .scroll-bar > .increment-button:pressed > .increment-arrow,
 .scroll-bar > .decrement-button:pressed > .decrement-arrow {
     -fx-background-color: -fx-mark-highlight-color, rgb(255, 255, 255);
-}
+}
+
+

racevisionGame/src/main/resources/css/nightMode.css

@@ -51,4 +51,9 @@
 .scroll-bar > .increment-button:pressed > .increment-arrow,
 .scroll-bar > .decrement-button:pressed > .decrement-arrow {
     -fx-background-color: -fx-mark-highlight-color, rgb(255, 255, 255);
-}
+}
+
+
+#arrowImage {
+    -fx-image: url("/visualiser/images/arrowLight.png");
+}