Flutter cluster dashboard UI improvements

After using the Flutter instrument cluster demo at ALS, a couple of things were noticed that could be tweaked to improve the UI for the demo:

• Most of the icons are a bit too dark, making them hard to make out.
• The connecting pop-up for the KUKSA.val connection if fine in the center of the screen if using the built-in navigation, but since that's going to need significant further development, it should be moved to the bottom of the screen and made smaller so it does not get obstructed by the streaming navigation receiver's window.