[HTC One X] geenyous:
Thanks, that was the issue, now it worked.

Also the signed SystemUI is loading just fine (I notice it as it has unthemed icons now).

But now I have the problem that I did not theme what I wanted to :-/

I wanted to change the color of the text on the toggle buttons from black/grey to white. Found the grey value in res/layout/status_bar_toggle_slider.xml - changed it from ff666666 to ffffffff but the text is still grey...

I also looked for something in the skin.apk, but I couldn't find it.

Does somebody happen to know where the right value has to be changed?

This is what the above mentioned xml looks like:

<?xml version="1.0" encoding="utf-8"?>
<merge
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui">
<CheckBox android:id="@id/toggle" android:layout_width="48.0dip" android:layout_height="0.0dip" android:button="@null" android:layout_alignParentLeft="true" android:layout_alignParentTop="true" android:layout_alignParentBottom="true" />
<com.htc.widget.HtcSeekBar android:id="@id/slider" android:paddingLeft="20.0dip" android:paddingRight="20.0dip" android:layout_width="0.0dip" android:layout_height="wrap_content" android:layout_toRightOf="@id/toggle" android:layout_alignParentRight="true" android:layout_centerVertical="true"
xmlns:seekbar="http://schemas.android.com/apk/res/com.htc" />
<TextView android:textSize="12.0sp" android:textColor="#ffffffff" android:gravity="center" android:id="@id/label" android:paddingTop="26.0dip" android:layout_width="0.0dip" android:layout_height="wrap_content" android:layout_alignLeft="@id/toggle" android:layout_alignRight="@id/toggle" android:layout_centerVertical="true" />
</merge>

j4n told us to look in status_bar_preferences,xml, but I don't know what I could change here that relates to the toggle button text:

<?xml version="1.0" encoding="utf-8"?>
<com.android.systemui.statusbar.PreferenceView android:orientation="vertical" android:focusable="true" android:clickable="true" android:layout_width="fill_parent" android:layout_height="wrap_content"
xmlns:android="http://schemas.android.com/apk/res/android">
<LinearLayout android:gravity="center_vertical" android:duplicateParentState="true" android:layout_width="fill_parent" android:layout_height="48.0dip">
<ImageView android:layout_gravity="center_vertical" android:id="@id/eqs_icon" android:layout_width="48.0dip" android:layout_height="48.0dip" />
<LinearLayout android:layout_gravity="center_vertical" android:orientation="vertical" android:duplicateParentState="true" android:layout_width="0.0dip" android:layout_height="wrap_content" android:layout_weight="1.0">
<TextView android:textSize="@com.htc:dimen/list_primary_m" android:textColor="@color/list_item_primary_text" android:id="@id/eqs_title" android:duplicateParentState="true" android:layout_width="fill_parent" android:layout_height="wrap_content" />
<TextView android:textSize="@com.htc:dimen/list_secondary_m" android:textColor="@color/list_item_secondary_text" android:id="@id/eqs_summary" android:duplicateParentState="true" android:layout_width="fill_parent" android:layout_height="wrap_content" />
<com.htc.widget.HtcSeekBar android:id="@id/seekbar" android:visibility="gone" android:duplicateParentState="true" android:layout_width="fill_parent" android:layout_height="wrap_content" />
</LinearLayout>
<com.htc.widget.HtcToggleButton android:layout_gravity="center_vertical" android:id="@id/eqs_togglebutton" android:clickable="false" android:duplicateParentState="true" android:layout_width="wrap_content" android:layout_height="wrap_content" />
</LinearLayout>
<View android:id="@id/notification_divider" android:background="@drawable/notification_divider" android:layout_width="fill_parent" android:layout_height="@dimen/notification_divider_height" android:layout_alignParentBottom="true" />
</com.android.systemui.statusbar.PreferenceView>

Thanks for your help!