@Brandon's answer would have worked, but I think this is a little more elegant:
<ComboBox Name="AvailableStyles">
<ComboBoxItem Tag="{x:Null}" IsSelected="True">None</ComboBoxItem>
<ComboBoxItem Tag="{StaticResource FirstStyle}" Style="{StaticResource FirstStyle}">Style 1</ComboBoxItem>
<ComboBoxItem Tag="{StaticResource SecondStyle}" Style="{StaticResource SecondStyle}">Style 2</ComboBoxItem>
<ComboBoxItem Tag="{StaticResource ThirdStyle}" Style="{StaticResource ThirdStyle}">Style 3</ComboBoxItem>
</ComboBox>
<Button Style="{Binding ElementName=AvailableStyles, Path=SelectedItem.Tag}" Content="Dynamically Styled Button" />
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