When you are overriding control template,you need to implement the whole default control template for that control.
Here's link: http://msdn.microsoft.com/en-us/library/ms753328%28v=vs.110%29.aspx
I am providing sample code which is working fine for me:
<Grid>
<Grid.Resources>
<SolidColorBrush x:Key="ControlBackground_MouseOver" Color="AliceBlue"/>
</Grid.Resources>
<Button Width="150" Height="50" Content="Hello" >
<Button.Template>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Border" Storyboard.TargetProperty="Background" Duration="0:0:0">
<DiscreteObjectKeyFrame KeyTime="0:0:0" Value="{StaticResource ControlBackground_MouseOver}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="Border"
BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}"
/>
<ContentControl x:Name="ContentElement"
IsTabStop="False"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
Foreground="{TemplateBinding Foreground}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding Padding}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
</ContentControl>
<Border
BorderThickness="1"
Opacity="0"
x:Name="FocusState"
/>
</Grid>
</ControlTemplate>
</Button.Template>
<Button.Style>
<Style TargetType="Button">
<Setter Property="Background" Value="Green"/>
</Style>
</Button.Style>
</Button>
</Grid>
If you want to only display text on button,you can just use content property of button.
Please let me know if this was what you were looking for .
Thanks.