[Sony Cross-Device Development Themes and Apps] Emperor.dk:
The theme explained

What ever wallpaper you set, will show in system apps as dimmed background.

For this to work, windowShowWallpaper has to be set to "true" for the default styles.

The default style for Xperia is Theme.DeviceDefault.Light. By luck windowShowWallpaper is not set in that style, but in the parrent style Theme.Holo.Light. This is great, because I then only have to keep style entries that are not set in Theme.DeviceDefault.Light.

I then just added Theme.Holo.Light to the themed styles.xml and set windowShowWallpaper has to be set to "true":

<item name="windowShowWallpaper">true</item>

.

The full styles.xml can be seen below:

<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="Widget.EditText" parent="@android:style/Widget">
<item name="android:textAppearance">@android:style/TextAppearance.Medium.Inverse</item>
<item name="android:textColor">@color/semc_edittext_color_dark</item>
<item name="android:textColorHint">@android:color/hint_foreground_holo_light</item>
<item name="android:textCursorDrawable">@null</item>
<item name="android:gravity">center_vertical</item>
<item name="android:background">@android:drawable/edit_text</item>
<item name="android:focusable">true</item>
<item name="android:focusableInTouchMode">true</item>
<item name="android:clickable">true</item>
</style>
<style name="TextAppearance.DeviceDefault.Widget.EditText" parent="@android:style/TextAppearance.DeviceDefault.Widget">
<item name="android:textColor">@color/semc_edittext_color_dark</item>
<item name="android:textColorHint">@android:color/hint_foreground_holo_light</item>
</style>
<style name="Theme.Holo.Light" parent="@android:style/Theme.Light">
<item name="android:colorForeground">@android:color/bright_foreground_holo_light</item>
<item name="android:colorBackground">@android:color/background_holo_light</item>
<item name="android:backgroundDimAmount">0.6</item>
<item name="android:disabledAlpha">0.5</item>
<item name="android:textColorPrimaryDisableOnly">@android:color/primary_text_disable_only_holo_light</item>
<item name="android:textColorSecondaryInverse">@android:color/secondary_text_holo_dark</item>
<item name="android:textColorPrimaryNoDisable">@android:color/primary_text_nodisable_holo_light</item>
<item name="android:textColorSecondaryNoDisable">@android:color/secondary_text_nodisable_holo_light</item>
<item name="android:textColorPrimaryInverseNoDisable">@android:color/primary_text_nodisable_holo_dark</item>
<item name="android:textColorSecondaryInverseNoDisable">@android:color/secondary_text_nodisable_holo_dark</item>
<item name="android:textColorHintInverse">@android:color/hint_foreground_holo_dark</item>
<item name="android:textCheckMark">@android:drawable/indicator_check_mark_light</item>
<item name="android:textCheckMarkInverse">@android:drawable/indicator_check_mark_dark</item>
<item name="android:galleryItemBackground">@android:drawable/gallery_item_background</item>
<item name="android:listPreferredItemHeight">64.0dip</item>
<item name="android:expandableListPreferredItemPaddingLeft">40.0dip</item>
<item name="android:expandableListPreferredChildPaddingLeft">?android:expandableListPreferredItemPaddingLeft</item>
<item name="android:expandableListPreferredItemIndicatorLeft">3.0dip</item>
<item name="android:expandableListPreferredItemIndicatorRight">0.0dip</item>
<item name="android:expandableListPreferredChildIndicatorLeft">?android:expandableListPreferredItemIndicatorLeft</item>
<item name="android:expandableListPreferredChildIndicatorRight">?android:expandableListPreferredItemIndicatorRight</item>
<item name="android:windowFrame">@null</item>
<item name="android:windowNoTitle">false</item>
<item name="android:windowIsFloating">false</item>
<item name="android:windowTitleSize">25.0dip</item>
<item name="android:panelFullBackground">@android:drawable/menu_background_fill_parent_width</item>
<item name="android:panelColorForeground">?android:textColorPrimary</item>
<item name="android:panelColorBackground">#ff000000</item>
<item name="android:panelTextAppearance">?android:textAppearance</item>
<item name="android:scrollbarSize">10.0dip</item>
<item name="android:scrollbarThumbHorizontal">@android:drawable/scrollbar_handle_holo_light</item>
<item name="android:scrollbarThumbVertical">@android:drawable/scrollbar_handle_holo_light</item>
<item name="android:scrollbarTrackHorizontal">@null</item>
<item name="android:scrollbarTrackVertical">@null</item>
<item name="android:spinnerStyle">?android:dropDownSpinnerStyle</item>
<item name="android:preferenceLayoutChild">@android:layout/preference_child_holo</item>
<item name="android:textColorHint">@android:color/hint_foreground_holo_light</item>
<item name="android:textColorLink">@android:color/holo_blue_light</item>
<item name="android:colorForegroundInverse">@android:color/bright_foreground_inverse_holo_light</item>
<item name="android:windowFullscreen">false</item>
<item name="android:textColorTertiary">@android:color/tertiary_text_holo_light</item>
<item name="android:textColorTertiaryInverse">@android:color/tertiary_text_holo_dark</item>
<item name="android:listDivider">@android:drawable/list_divider_holo_light</item>
<item name="android:windowSoftInputMode">stateUnspecified|adjustUnspecified</item>
<item name="android:candidatesTextStyleSpans">@android:string/candidates_style</item>
<item name="android:textColorSearchUrl">@android:color/search_url_text_holo</item>
<item name="android:textColorPrimaryInverseDisableOnly">@android:color/primary_text_disable_only_holo_dark</item>
[COLOR="Red"]<item name="android:windowShowWallpaper">true</item>[/COLOR]
<item name="android:scrollbarFadeDuration">250</item>
<item name="android:scrollbarDefaultDelayBeforeFade">300</item>
<item name="android:colorBackgroundCacheHint">@android:drawable/background_cache_hint_selector_holo_light</item>
<item name="android:textSelectHandleLeft">@android:drawable/text_select_handle_left</item>
<item name="android:textSelectHandleRight">@android:drawable/text_select_handle_right</item>
<item name="android:textSelectHandle">@android:drawable/text_select_handle_middle</item>
<item name="android:windowActionBar">true</item>
<item name="android:windowActionModeOverlay">false</item>
<item name="android:actionBarSize">@android:dimen/action_bar_default_height</item>
<item name="android:listDividerAlertDialog">@android:drawable/list_divider_holo_light</item>
<item name="android:textColorAlertDialogListItem">@android:color/primary_text_holo_light</item>
<item name="android:dividerVertical">?android:listDivider</item>
<item name="android:dividerHorizontal">?android:listDivider</item>
<item name="android:fastScrollTrackDrawable">@android:drawable/fastscroll_track_holo_light</item>
<item name="android:textColorHighlightInverse">@android:color/highlighted_text_holo_dark</item>
<item name="android:textColorLinkInverse">@android:color/holo_blue_light</item>
<item name="android:textCursorDrawable">@android:drawable/text_cursor_holo_light</item>
<item name="android:listPreferredItemHeightLarge">80.0dip</item>
<item name="android:listPreferredItemHeightSmall">48.0dip</item>
<item name="android:colorPressedHighlight">@android:color/holo_gray_light</item>
<item name="android:colorLongPressedHighlight">@android:color/holo_gray_bright</item>
<item name="android:colorFocusedHighlight">@android:color/holo_blue_dark</item>
<item name="android:colorActivatedHighlight">@android:color/holo_blue_dark</item>
<item name="android:colorMultiSelectHighlight">@android:color/holo_green_light</item>
<item name="android:actionModeStyle">@android:style/Widget.Holo.Light.ActionMode</item>
<item name="android:actionBarWidgetTheme">@null</item>
<item name="android:actionModeSplitBackground">@android:drawable/cab_background_bottom_holo_light</item>
<item name="android:textAppearanceListItemSmall">?android:textAppearanceMedium</item>
<item name="android:listPreferredItemPaddingLeft">8.0dip</item>
<item name="android:listPreferredItemPaddingRight">8.0dip</item>
<item name="android:searchWidgetCorpusItemBackground">@android:color/search_widget_corpus_item_background</item>
<item name="android:mediaRouteButtonStyle">@android:style/Widget.Holo.Light.MediaRouteButton</item>
<item name="android:listPreferredItemPaddingStart">8.0dip</item>
<item name="android:listPreferredItemPaddingEnd">8.0dip</item>
<item name="android:presentationTheme">@android:style/Theme.Holo.Light.Dialog.Presentation</item>
<item name="android:checkedTextViewStyle">@android:style/Widget.Holo.Light.CheckedTextView</item>
<item name="android:windowOverscan">false</item>
<item name="android:errorMessageBackground">@android:drawable/popup_inline_error_holo_light</item>
<item name="android:errorMessageAboveBackground">@android:drawable/popup_inline_error_above_holo_light</item>
<item name="android:searchResultListItemHeight">58.0dip</item>
<item name="android:dropdownListPreferredItemHeight">?android:listPreferredItemHeightSmall</item>
<item name="android:alertDialogCenterButtons">false</item>
<item name="android:panelMenuIsCompact">true</item>
<item name="android:panelMenuListWidth">250.0dip</item>
<item name="android:panelMenuListTheme">@android:style/Theme.Holo.Light.CompactMenu</item>
<item name="android:gestureOverlayViewStyle">@android:style/Widget.Holo.Light.GestureOverlayView</item>
<item name="android:stackViewStyle">@android:style/Widget.Holo.StackView</item>
<item name="android:numberPickerStyle">@android:style/Widget.Holo.Light.NumberPicker</item>
<item name="android:timePickerStyle">@android:style/Widget.Holo.Light.TimePicker</item>
<item name="android:activityChooserViewStyle">@android:style/Widget.Holo.Light.ActivityChooserView</item>
<item name="android:preferenceFragmentStyle">@android:style/PreferenceFragment.Holo</item>
<item name="android:searchDropdownBackground">@android:drawable/search_dropdown_light</item>
<item name="android:preferenceFrameLayoutStyle">@android:style/Widget.Holo.PreferenceFrameLayout</item>
</style>
</resources>

All the semi-transparent PNGs were made from the stock dark and light PNGs from stock firmware with Pixlr Editor.

I cut out the part that needed to be semi-transparent with Wand or Marquee tool, paste to new layer and set transparancy to 50%, and then Merge down to one layer.

Take good care when editing .9-PNGs, read more in my oooold thread here.

I think the Light version needs to be 25% (that more transparent, 100% = no transparency).