WPF TabItem Content alignment is always centered

Question

I'm trying to put some content inside my TabItem, but I can't add a container that fills the space of the TabItem. Anything that I add shows up centered.

I'm using a Theme from WPF Toolkit (WhistlerBlue.xaml), so I think that maybe it's that what is causing me this issue.

I cannot remove the Theme, because I need it. At most I could change something in it, but I'm new to WPF, and don't know what should I be changing.

My TabControl looks like this:

<TabControl Margin="0,71.25,0,206.25" Name="tabControl1" TabStripPlacement="Left">
    <TabItem Name="tabItem1" FlowDirection="LeftToRight" FontSize="22" Height="200" Width="60" >
        <TabItem.Header>
            <StackPanel Orientation="Horizontal" >
                <Image Height="40" Width="40" Margin="20,0,0,0" VerticalAlignment="Center"></Image>
                <TextBlock Margin="15,0,25,0" VerticalAlignment="Center" FontWeight="Bold">
                    Header
                </TextBlock>
            </StackPanel>
        </TabItem.Header>
        <TabItem.Content>
            <StackPanel HorizontalAlignment="Left" VerticalAlignment="Top" Margin="0,0,0,0" >
                <TextBlock>Test</TextBlock>
            </StackPanel>
        </TabItem.Content>
    </TabItem>
</TabControl>

There is similar question, but it don't has the answer: In WPF, how do I get the content of a tabItem to fill available space?

Can someone help me? Thanks.

Answer 1

Ok, I found the answer.

The .xaml of the theme started with these settings for TabItem:

<Style d:IsControlPart="True" TargetType="{x:Type TabItem}">
    <Setter Property="HorizontalContentAlignment" Value="Center"/>
    <Setter Property="VerticalContentAlignment" Value="Center"/>

I've changed Center to Stretch, and the problem was solved.

I really need to learn WPF from a book, instead of just trying things out.