Improve Visibility of Chat Notification Flash

[Phergus]

Is your feature request related to a problem? Please describe.
On Windows 10 with the default color scheme the task bar is very dark and the chat notification flash from the MapTool taskbar icon may not be very obvious.

Describe the solution you'd like
The notification flash uses a pure blue (0x0000FF) but should probably use a contrasting color(s) to make it more obvious on dark color schemes.

Additional context
https://forums.rptools.net/viewtopic.php?f=26&t=28391

[Phergus]

Closing.