Mahapps Metro - Expander: applying LayoutTrasform to header changes style

Question

In a WPF application using Mahapps Metro, I'd like to use an Expander on the right. To prevent the Expander to take too much space when collapsed, I'd like to apply a layoutTrasform to its header.

Without applying any rotation:

        <Expander
            DockPanel.Dock="Right"
            ExpandDirection="Right">
            <Expander.Header>        
                    Options
            </Expander.Header>

the expander is shown like this:

Applying the rotation:

<Expander
            DockPanel.Dock="Right"
            ExpandDirection="Right">
            <Expander.Header>
                <TextBlock>
                    <TextBlock.LayoutTransform>
                        <RotateTransform Angle="90" />
                    </TextBlock.LayoutTransform>
                    Options
                </TextBlock>
            </Expander.Header>

the header style changes to this:

How can I apply the rotation, maintaining the correct header style?