Android theme not being set

I have a theme that refuses to be applied to activities - none of the styles are applied. If I don't supply the layout_width/layout_height attributes for <Button> it also gets a runtime error, showing that the Button class isn't being applied.

/res/values/themes.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="Theme" parent="android:style/Theme.Black">
        <item name="android:windowNoTitle">true</item>
        <item name="android:buttonStyle">@style/Button</item>
        <item name="android:windowBackground">@color/page_background_light</item>
        <item name="android:textAppearance">@style/TextAppearance</item>
    </style>
</resources>

/res/values/styles.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="TextAppearance" parent="@android:style/TextAppearance">
        <item name="android:textSize">12sp</item>
        <item name="android:textColor">@color/darkblue</item>
    </style>
    <style name="Button" parent="@android:style/Widget.Button">
        <item name="android:layout_width">fill_parent</item> 
        <item name="android:layout_height">wrap_content</item>
        <!--<item name="android:textColor">#3C2F4F</item>
        <item name="android:textSize">20dip</item>-->
    </style>
</resources>

and the relevant manifest setting:

<application android:icon="@drawable/icon"
    android:label="@string/app_name"
    android:theme="@style/Theme">

What's the obvious mistake I'm missing?