LevelDialog.xaml

<Window x:Class="TeachersMate.LevelDialog"
        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:TeachersMate"
        mc:Ignorable="d"
      Title="Select book data" SizeToContent="WidthAndHeight" Background="#F2E5E5">
    <StackPanel Orientation="Vertical" Margin="10">
        <TextBlock Text="Select Level:" FontFamily="Roboto" Foreground="#665A48" FontSize="17"/>
        <ComboBox Style="{StaticResource MaterialDesignComboBox}" FontFamily="Roboto"  Foreground="#454545" FontSize="17" Height="35" Name="cmbLevel" Margin="10">
            <ComboBoxItem>A1</ComboBoxItem>
            <ComboBoxItem>A2</ComboBoxItem>
            <ComboBoxItem>B1</ComboBoxItem>
            <ComboBoxItem>B2</ComboBoxItem>
            <ComboBoxItem>C1</ComboBoxItem>
            <ComboBoxItem>C2</ComboBoxItem>
        </ComboBox>
        <StackPanel Orientation="Horizontal" Margin="10,20,0,0">
            <TextBlock FontFamily="Roboto" Foreground="#665A48" FontSize="17" Text="Author:" Margin="0,0,10,0"/>
            <TextBox Name="txtAuthor" FontFamily="Roboto"  Foreground="#454545"  FontSize="17" Height="30" Width="290" Margin="40,0,10,0"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="10,20,0,10">
            <TextBlock FontFamily="Roboto" Foreground="#665A48" FontSize="17" Text="New file name:" Margin="0,0,10,0" VerticalAlignment="Center"/>
            <TextBox Name="txtFileName" FontFamily="Roboto"  Foreground="#454545"  Width="150" Margin="0,0,10,0" FontSize="17" Height="30"/>
            <Button Name="btnCancel"  FontFamily="Roboto" Content="Cancel" Margin="10" Width="80" Click="btnCancel_Click"/>
            <Button Name="btnOk"  FontFamily="Roboto" Content="Ok" Margin="10" Width="80" Click="btnOk_Click"/>
        </StackPanel>
    </StackPanel>
</Window>