NumberSystem/numbersystem/MainWindow.xaml

<Window x:Class="numbersystem.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:numbersystem"
        mc:Ignorable="d"
        Title="Система счислений" Height="550" Width="350" ResizeMode="NoResize" Icon="/icon.png" FontFamily="Montserrat Light">
    <Window.Resources>
        <Style TargetType="Button">
            <Style.Setters>
                <Setter Property="ItemsControl.Background" Value="#8888d7"/>
                <Setter Property="ItemsControl.Foreground" Value="White"/>
                <Setter Property="ItemsControl.Margin" Value="10"/>
            </Style.Setters>
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter Property="Background" Value="White" />
                    <Setter Property="Foreground" Value="Black" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <Grid>
        <TabControl>
            <TabItem Header="Калькулятор" FontFamily="Montserrat Light">
                <Grid Background="#d7d7f2">
                    <Grid.RowDefinitions>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                        <RowDefinition/>
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                    </Grid.ColumnDefinitions>
                    <TextBox x:Name="str" IsReadOnly="True" Grid.Row="0" Grid.ColumnSpan="4" Margin="10" 
                             FontSize="16" HorizontalScrollBarVisibility="Auto" 
                             VerticalContentAlignment="Center" HorizontalContentAlignment="Right"
                             PreviewTextInput="str_PreviewTextInput"
                             KeyDown="str_KeyDown" PreviewKeyDown="str_KeyDown"/>
                    <Label Content="Выберите СС:" Grid.Row="1" Grid.ColumnSpan="2" Margin="10,10,43,10" FontSize="14"/>
                    <ComboBox x:Name="comboBox" Margin="48,10,10,30" Grid.ColumnSpan="3" Grid.Column="1" Grid.Row="1" 
                              SelectionChanged="ComboBox_SelectionChanged">
                        <ComboBoxItem Content="Двоичная"/>
                        <ComboBoxItem Content="Восьмеричная"/>
                        <ComboBoxItem Content="Шестнадцатиричная"/>
                    </ComboBox>
                    <Button x:Name="ca" 
                            Content="CA" Grid.Row="5" Grid.Column="0"/>
                    <Button x:Name="zero" 
                            Content="0" Grid.Row="5" Grid.Column="1"/>
                    <Button x:Name="erase" 
                            Content="⌫" Grid.Row="5" Grid.Column="2"/>
                    <Button x:Name="one" 
                            Content="1" Grid.Row="4" Grid.Column="0"/>
                    <Button x:Name="two" 
                            Content="2" Grid.Row="4" Grid.Column="1"/>
                    <Button x:Name="three" 
                            Content="3" Grid.Row="4" Grid.Column="2"/>
                    <Button x:Name="four" 
                            Content="4" Grid.Row="3" Grid.Column="0"/>
                    <Button x:Name="five" 
                            Content="5" Grid.Row="3" Grid.Column="1"/>
                    <Button x:Name="six" 
                            Content="6" Grid.Row="3" Grid.Column="2"/>
                    <Button x:Name="seven" 
                            Content="7" Grid.Row="2" Grid.Column="0"/>
                    <Button x:Name="eight" 
                            Content="8" Grid.Row="2" Grid.Column="1"/>
                    <Button x:Name="nine" 
                            Content="9" Grid.Row="2" Grid.Column="2"/>
                    <Button x:Name="a" 
                            Content="A" Grid.Row="6" Grid.Column="0" Background="#6c6cdf"/>
                    <Button x:Name="b" 
                            Content="B" Grid.Row="6" Grid.Column="1" Background="#6c6cdf"/>
                    <Button x:Name="c" 
                            Content="C" Grid.Row="6" Grid.Column="2" Background="#6c6cdf"/>
                    <Button x:Name="d" 
                            Content="D" Grid.Row="7" Grid.Column="0" Background="#6c6cdf"/>
                    <Button x:Name="e" 
                            Content="E" Grid.Row="7" Grid.Column="1" Background="#6c6cdf"/>
                    <Button x:Name="f" 
                            Content="F" Grid.Row="7" Grid.Column="2" Background="#6c6cdf"/>
                    <Button x:Name="plus" 
                            Content="+" Grid.Row="2" Grid.Column="3" Background="#4040bf"/>
                    <Button x:Name="minus" 
                            Content="-" Grid.Row="3" Grid.Column="3" Background="#4040bf"/>
                    <Button x:Name="multiply" 
                            Content="×" Grid.Row="4" Grid.Column="3" Background="#4040bf"/>
                    <Button x:Name="division" 
                            Content="÷" Grid.Row="5" Grid.Column="3" Background="#4040bf"/>
                    <Button x:Name="remains" 
                            Content="%" Grid.Row="6" Grid.Column="3" Background="#4040bf"/>
                    <Button x:Name="equal" 
                            Content="=" Grid.Row="7" Grid.Column="3" Background="#4040bf"/>
                </Grid>
            </TabItem>
            <TabItem Header="История" FontFamily="Montserrat Light">
                <Grid Background="#d7d7f2"/>
            </TabItem>
            <TabItem Header="Диаграмма" FontFamily="Montserrat Light">
                <Grid Background="#d7d7f2"/>
            </TabItem>
        </TabControl>

    </Grid>
</Window>