OS.setTheme (isDark) seems to trigger the OS to use the correct arrow
colors. So we can retire our custom drawing on this. This removes the
CSS but a later commit could also retire the custom drawing operation.

Change-Id: I80c097419066c30c8395ac446f7381faae754e5a
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>