How can I set different fonts for tabitem header and content of this tabitem??
Like anything in WPF, there are many ways. Without knowing exactly what you are trying to do, here is one "for instance" (I wouldn't suggest using this combination of fonts:) )
<TabControl>
<TabControl.Resources>
<Style TargetType="{x:Type TextBlock}">
<Setter Property="FontFamily" Value="Comic Sans MS" />
<Setter Property="FontSize" Value="20" />
</Style>
<Style x:Key="headerStyle" TargetType="{x:Type TextBlock}">
<Setter Property="Control.FontFamily" Value="Papyrus" />
<Setter Property="Control.FontSize" Value="12" />
</Style>
</TabControl.Resources>
<TabItem>
<TabItem.Header>
<TextBlock Text="Header" Style="{StaticResource headerStyle}" />
</TabItem.Header>
<TextBlock Text="Here is the content" />
</TabItem>
</TabControl>