[Android Themes] jairomeo:
Inside the source of the calendarproviders there is a colors.xml that gives you

<resources>
<color name="appwidget_date">#ff666666</color>
<color name="appwidget_date_shadow">#ffffffff</color>

<color name="appwidget_when">#ff000000</color>
<color name="appwidget_title">#ff000000</color>
<color name="appwidget_where">#ff000000</color>
<color name="appwidget_conflict">#ff666666</color>

<color name="appwidget_no_events">#bb000000</color>
</resources>

And the agenda_appwidget.xml references the values inside the colors.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/agenda_appwidget"
android:layout_width="@dimen/appwidget_width"
android:layout_height="@dimen/appwidget_height"
android:background="@drawable/appwidget_bg">

<TextView
android:id="@+id/no_events"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dip"
android:gravity="center"
android:textSize="14sp"
android:textStyle="bold"
android:textColor="@color/appwidget_no_events"
android:text="@string/gadget_no_events"
/>

<TextView
android:id="@+id/day_of_month"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignParentRight="true"
android:layout_marginRight="7dip"
android:textSize="30sp"
android:textStyle="bold"
android:textColor="@color/appwidget_date"
android:singleLine="true"
android:shadowColor="@color/appwidget_date_shadow"
android:shadowDy="1"
android:shadowRadius="0.9"
android:visibility="gone"
/>

<TextView
android:id="@+id/day_of_week"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_toLeftOf="@id/day_of_month"
android:layout_alignBottom="@id/day_of_month"
android:layout_alignBaseline="@id/day_of_month"
android:layout_marginLeft="7dip"
android:textSize="17sp"
android:textStyle="bold"
android:textColor="@color/appwidget_date"
android:singleLine="true"
android:shadowColor="@color/appwidget_date_shadow"
android:shadowDy="1"
android:shadowRadius="0.9"
android:visibility="gone"
/>

<ImageView
android:id="@+id/divider"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_below="@id/day_of_week"
android:layout_marginTop="2dip"
android:layout_marginLeft="3dip"
android:layout_marginRight="3dip"
android:background="@drawable/white_list_rule_cal"
android:visibility="gone"
/>

<TextView
android:id="@+id/when"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_below="@id/divider"
android:layout_marginTop="5dip"
android:layout_marginLeft="7dip"
android:layout_marginRight="7dip"
android:textSize="14sp"
android:textStyle="bold"
android:textColor="@color/appwidget_when"
android:singleLine="true"
android:visibility="gone"
/>

<TextView
android:id="@+id/where"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_marginBottom="4dip"
android:layout_marginLeft="7dip"
android:layout_marginRight="7dip"
android:textSize="11sp"
android:textColor="@color/appwidget_where"
android:maxLines="2"
android:visibility="gone"
/>

<TextView
android:id="@+id/conflict"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_above="@id/where"
android:layout_alignWithParentIfMissing="true"
android:layout_marginBottom="4dip"
android:layout_marginLeft="7dip"
android:layout_marginRight="7dip"
android:textSize="11sp"
android:textColor="@color/appwidget_conflict"
android:singleLine="true"
android:visibility="gone"
/>

<TextView
android:id="@+id/title"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_below="@id/when"
android:layout_above="@id/conflict"
android:layout_alignWithParentIfMissing="true"
android:layout_marginTop="3dip"
android:layout_marginBottom="4dip"
android:layout_marginLeft="7dip"
android:layout_marginRight="7dip"
android:textSize="14sp"
android:textStyle="bold"
android:textColor="@color/appwidget_title"
android:maxLines="4"
android:visibility="gone"
/>

</RelativeLayout>

So you'd just have to create your own CalendarProvider.apk from the source.