您为Foreground属性（Foreground = Brushes.Red）分配了本地值。之后可能会触发样式触发器，但触发器中的Foreground setter将被忽略，因为本地值具有更大的权重（它被称为Dependency Property value precedence）
要修复该行为，请通过样式设置器设置正常前景值：

var foregroundNormalSetter = new Setter()
{
 Property = Button.ForegroundProperty,
 Value = Brushes.Red
};

var foregroundSetter = new Setter()
{
 Property = Button.ForegroundProperty,
 Value = Brushes.LightGray
};

var borderThicknessSetter = new Setter()
{
 Property = Button.BorderThicknessProperty,
 Value = new Thickness(0)
};

var isEnabledTrigger = new Trigger()
{
 Property = Button.IsEnabledProperty,
 Value = false
 //Setters = { foregroundSetter, borderThicknessSetter }
};
isEnabledTrigger.Setters.Add(foregroundSetter);
isEnabledTrigger.Setters.Add(borderThicknessSetter);

var style = new Style();
style.Setters.Add(foregroundNormalSetter);
style.Triggers.Add(isEnabledTrigger);

var pdfButton = new Button()
{
 Padding = new Thickness(0, 0, 0, 0),
 Margin = new Thickness(0, 0, 5, 0),
 Height = 18,
 Width = 20,
 Content = "PDF",
 FontSize = 9,
 FontWeight = FontWeights.SemiBold,
 //Foreground = Brushes.Red,
 BorderBrush = Brushes.Transparent,
 Background = Brushes.Transparent,
 ToolTip = new ToolTip() { Content = "Create PDF of training day." },
 Command = CreatePdfCommand,
 CommandParameter = newTab,
 Style = style
};

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