Respect Console.BackGround color

This was bugging me. Instead of using the existing console's background
color, we were hard-coding "Black." Now we just leave it alone.

src/Microsoft.PowerShell.Linux.Host/main.cs

@@ -518,7 +518,7 @@ OPTIONS
                     prompt = "PS> ";
                 }
 
-                this.myHost.UI.Write(ConsoleColor.White, ConsoleColor.Black, prompt);
+                this.myHost.UI.Write(ConsoleColor.White, Console.BackgroundColor, prompt);
                 string cmd = consoleReadLine.Read(this.myHost.Runspace);
                 this.Execute(cmd);
             }