Remove padding from ComboBox - xaml

Question

I'm having trouble with removing the padding in the list of a ComboBox.
I filled the list with three buttons, and above the first button there is space and below the last button there is also space:
When closed
When opened
Another problem is that the list doesn't appear beneath the button, as you can see on the second picture.
This is the code (note there is no styling yet):
<ComboBox x:Name="importComboBox" Margin="25" HorizontalAlignment="Stretch">
<ComboBox.PlaceholderText>Placeholder</ComboBox.PlaceholderText>
<ComboBoxItem Padding="0" Margin="0">
<Button VerticalAlignment="Stretch" HorizontalAlignment="Stretch">button 1</Button>
</ComboBoxItem>
<ComboBoxItem Padding="0" Margin="0">
<Button VerticalAlignment="Stretch" HorizontalAlignment="Stretch">button 2</Button>
</ComboBoxItem>
<ComboBoxItem Padding="0" Margin="0">
<Button VerticalAlignment="Stretch" HorizontalAlignment="Stretch">button 3</Button>
</ComboBoxItem>
</ComboBox>
How can I remove those spaces and can I alter the dropdown position?

Answer 1

As Peter said, to achieve this we can modify the template of ComboBox. To modify the template of ComboBox, we can select the "[importComboBox]" in "Document Outline" and right click, then select "Edit Template" → "Edit a Copy...".
To remove the padding in the list. Editor the margin property of ItemsPresenter. ItemsPresenter specifies where items are placed in a control.
<ItemsPresenter Margin="0,0,0,0" />
To make the list appear beneath the button, we can editor the Margin property of Border in Popup. It determines the location of Popup.
<Border x:Name="PopupBorder"
BorderBrush="{ThemeResource SystemControlForegroundChromeHighBrush}"
BorderThickness="{ThemeResource ComboBoxDropdownBorderThickness}"
Background="{ThemeResource SystemControlBackgroundChromeMediumLowBrush}"
HorizontalAlignment="Stretch"
Margin="0,32,0,0">