The @ianhanniballake's answer is absolutely correct and simple. But it took me few days to understand. For someone who don't understand his answer, here is more detail implementation
<Button
android:id="@+id/btn"
style="@style/MaterialButton"
... />
<style name="MaterialButton" parent="Widget.AppCompat.Button.Colored">
<item name="android:theme">@style/Theme.MaterialButton</item>
...
</style>
<style name="Theme.MaterialButton" parent="YourTheme">
<item name="colorAccent">@color/yourAccentColor</item>
<item name="colorButtonNormal">@color/yourButtonNormalColor</item>
</style>
===Or===
<Button
android:id="@+id/btn"
style="@style/Widget.AppCompat.Button.Colored"
android:theme="@style/Theme.MaterialButton" />
<style name="Theme.MaterialButton" parent="YourTheme">
<item name="colorAccent">@color/yourAccentColor</item>
<item name="colorButtonNormal">@color/yourButtonNormalColor</item>
</style>