How do I change the background of a TextBox?

Try this:

gTBxComment.Background = Brushes.LightBlue; // Replace with your desired color
gTBxComment.Background = new SolidColorBrush(Color.FromRgb(255, 255, 204)); // Example: Light Yellow
gTBxComment.Background = new SolidColorBrush((Color)ColorConverter.ConvertFromString("#FFB6C1")); // Example: Light Pink

You try this

gTBxComment.Background = new SolidColorBrush(Color.FromRgb(255, 192, 203)); // Example: Light Pink

or try this

gTBxComment.Background = new SolidColorBrush(Colors.LightBlue); // Predefined color

Explanation

1. SolidColorBrush: Used to define a solid fill color.
2. Color.FromRgb: Allows you to specify the RGB values for the color.
3. Colors: Provides a list of predefined colors.