Hello,
The ToolWindowContainer control has TitleBarFontFamily and TitleBarFontSize properties. Those properties are set via default Style setters so you can't really change them per ToolWindow.
Something like this, which shows our default Setter values, in your App.Resources will alter all ToolWindow titles:
<Style TargetType="docking:ToolWindowContainer">
<Setter Property="TitleFontFamily" Value="{DynamicResource {x:Static SystemFonts.MessageFontFamilyKey}}" />
<Setter Property="TitleFontSize" Value="{DynamicResource {x:Static themes:AssetResourceKeys.ToolWindowContainerTitleFontSizeDoubleKey}}" />
</Style>
You also could alternatively override any of those resource keys with new value in your App.Resources instead.
Floating document windows are hosted in a Window that uses our WindowChrome. WindowChrome uses our AssetResourceKeys.WindowTitleFontSizeDoubleKey resource value for the title bar font size. WindowChrome doesn't have a resource for font family though.