components: Use background accent color for map marker

Since we only support maps in light mode, the accent color is too light
in dark mode so the marker doesn't have enough contrast with an "earth"
background, and even less with a "water" background.

The background accent color doesn't change between light and dark mode
and has enough contrast.

data/resources/stylesheet/_components.scss

@@ -70,7 +70,7 @@ media-content-viewer controls {
 }
 
 location-viewer .map-marker {
-  color: var(--accent-color);
+  color: var(--accent-bg-color);
 }
 
 entry inline-pill {