replace 0x00000000 with TerminalColor.transparent

lib/terminal/sgr.dart

@@ -4,15 +4,6 @@ import 'package:xterm/terminal/csi.dart';
 import 'package:xterm/terminal/terminal.dart';
 import 'package:xterm/utli/lookup_table.dart';
 
-// reference to color
-// class Cr implements TerminalColor {
-//   Cr(this.getter);
-
-//   final TerminalColor Function() getter;
-
-//   int get value => getter().value;
-// }
-
 /// SGR selects one or more character attributes at the same time.
 /// Multiple params (up to 32) are applied from in order from left to right.
 /// The changed attributes are applied to all new characters received.

lib/terminal/terminal.dart

@@ -14,6 +14,7 @@ import 'package:xterm/terminal/cursor.dart';
 import 'package:xterm/terminal/platform.dart';
 import 'package:xterm/terminal/sbc.dart';
 import 'package:xterm/terminal/tabs.dart';
+import 'package:xterm/theme/terminal_color.dart';
 import 'package:xterm/theme/terminal_theme.dart';
 import 'package:xterm/theme/terminal_themes.dart';
 import 'package:xterm/utli/debug_handler.dart';
@@ -47,7 +48,7 @@ class Terminal with Observable {
 
     cursor = Cursor(
       fg: theme.foreground,
-      bg: 0x00000000, // transparent
+      bg: TerminalColor.transparent, // transparent
       flags: 0x00, // no flags
     );