Fix text color visibility in dark theme dialogs

Add explicit white70 color to dark theme text styles to ensure text is visible against dark dialog backgrounds. This matches the pattern used in light theme where black87 is explicitly set.

Co-authored-by: rustdesk <71636191+rustdesk@users.noreply.github.com>

flutter/lib/common.dart

@@ -501,10 +501,10 @@ class MyTheme {
           )
         : null,
     textTheme: const TextTheme(
-      titleLarge: TextStyle(fontSize: 19),
-      titleSmall: TextStyle(fontSize: 14),
-      bodySmall: TextStyle(fontSize: 12, height: 1.25),
-      bodyMedium: TextStyle(fontSize: 14, height: 1.25),
+      titleLarge: TextStyle(fontSize: 19, color: Colors.white70),
+      titleSmall: TextStyle(fontSize: 14, color: Colors.white70),
+      bodySmall: TextStyle(fontSize: 12, color: Colors.white70, height: 1.25),
+      bodyMedium: TextStyle(fontSize: 14, color: Colors.white70, height: 1.25),
       labelLarge: TextStyle(
         fontSize: 16.0,
         fontWeight: FontWeight.bold,