Как изменить стиль кнопки для отключенных кнопок WPF?

Следующий код - это просто образец, основанный на вашем вопросе. У меня не реализована логика игры в крестики-нолики. Он демонстрирует, как реализовать общий стиль для кнопки, а также внедрение общего ClickEvent. Вы можете использовать его и менять по своему усмотрению.

XAML

<Window x:Class = "WPFTest.MainWindow"
        xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x = "http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d = "http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc = "http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local = "clr-namespace:WPFTest"
        mc:Ignorable = "d"
        Title = "TestWPF" Height = "300" Width = "400" 
        WindowStyle = "None" WindowStartupLocation = "CenterScreen">

    <Window.Resources>
        <Style x:Key = "buttonStyle" TargetType = "Button">

        <Setter Property = "OverridesDefaultStyle" Value = "True" />
        <Setter Property = "Background" Value = "MediumAquamarine" />
        <Setter Property = "Foreground" Value = "MediumBlue" />

        <Setter Property = "Template">
            <Setter.Value>
                <ControlTemplate TargetType = "{x:Type Button}">
                    <Grid Background = "{TemplateBinding Background}">
                        <ContentPresenter x:Name = "MyContentPresenter" 
                                      Content = "{TemplateBinding Content}"
                                      HorizontalAlignment = "Center" 
                                      VerticalAlignment = "Center" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>

        <Style.Triggers>
            <Trigger Property = "IsEnabled" Value = "False">
                <Setter Property = "Opacity" Value = "1"/>
                <Setter Property = "Foreground" Value = "Black"/>
                <Setter Property = "Background" Value = "LightGray"/>
            </Trigger>
        </Style.Triggers>
    </Style>
    </Window.Resources>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height = "*"/>
            <RowDefinition Height = "*"/>
            <RowDefinition Height = "*"/>
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width = "*"/>
            <ColumnDefinition Width = "*"/>
            <ColumnDefinition Width = "*"/>
        </Grid.ColumnDefinitions>

        <Button x:Name = "btn1" Grid.Row = "0" Grid.Column = "0" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn2" Grid.Row = "0" Grid.Column = "1" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn3" Grid.Row = "0" Grid.Column = "2" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>

        <Button x:Name = "btn4" Grid.Row = "1" Grid.Column = "0" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn5" Grid.Row = "1" Grid.Column = "1" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn6" Grid.Row = "1" Grid.Column = "2" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>

        <Button x:Name = "btn7" Grid.Row = "2" Grid.Column = "0" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn8" Grid.Row = "2" Grid.Column = "1" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>
        <Button x:Name = "btn9" Grid.Row = "2" Grid.Column = "2" Style = "{StaticResource buttonStyle}" Click = "btn_Click"/>

    </Grid>
</Window>

CS

using System;
using System.Windows;
using System.Windows.Controls;

namespace WPFTest
{
    public partial class MainWindow : Window
    {
        string sign = "X";
        public MainWindow()
        {
            InitializeComponent();
        }

        private void btn_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                Button currentButton = sender as Button;
                currentButton.Content = sign;
                currentButton.IsEnabled = false;

                swapSign();
            }
            catch (Exception ex)
            {
            }
        }

        private void swapSign()
        {
            if (sign == "X")
                sign = "O";
            else
                sign = "X";
        }
    }
}

ОБНОВЛЕНИЕ-1 Я изменил стиль в XAML. Проверить выше. Здесь следует отметить, что вы должны установить свойства Background и Foreground для ButtonStyle, как я установил.