How can I customize the Action Mode's color and text?

Updated answer for both pre- and post-Lollipop devices. You must remove the android: prefix to get it to work on Lollipop+ devices, like so:

styles.xml:

<style name="Widget.ActionMode">
    <item name="android:background">?android:attr/actionModeBackground</item>
    <item name="android:backgroundSplit">?android:attr/actionModeSplitBackground</item>
    <item name="android:height">?android:attr/actionBarSize</item>
    <item name="android:titleTextStyle">@android:style/TextAppearance.Widget.ActionMode.Title</item>
    <item name="android:subtitleTextStyle">@android:style/TextAppearance.Widget.ActionMode.Subtitle</item>
</style>

v21/styles.xml:

<style name="Widget.ActionMode">
    <item name="background">?android:attr/actionModeBackground</item>
    <item name="backgroundSplit">?android:attr/actionModeSplitBackground</item>
    <item name="height">?android:attr/actionBarSize</item>
    <item name="titleTextStyle">@android:style/TextAppearance.Widget.ActionMode.Title</item>
    <item name="subtitleTextStyle">@android:style/TextAppearance.Widget.ActionMode.Subtitle</item>
</style>

I'd also recommend having your style with parent="@style/Widget.AppCompat.ActionMode" set, so you inherit the attributes you don't care about overriding.