[Omnia 7 Windows Phone 7 Development] voluptuary:
if you are having trouble getting the light theme changed to dark via cab update I think I can help:

<wap-provisioningdoc>

<!-- custom 'light' theme -->

<characteristic type="Registry">

<characteristic type="HKLM\ControlPanel\Themes\0">
<parm name="SliderThumbDisabled" value="res://UIXMobile!ui.dark.slider.thumb.disabled.png" datatype="string" />
<parm name="SliderThumb" value="res://UIXMobile!ui.dark.slider.thumb.png" datatype="string" />
<parm name="ToggleSwitchTrackDisabled" value="res://UIXMobile!ui.dark.toggleswitch.track.disabled.png" datatype="string" />
<parm name="ToggleSwitchTrackPressed" value="res://UIXMobile!ui.dark.toggleswitch.track.down.png" datatype="string" />
<parm name="ToggleSwitchTrack" value="res://UIXMobile!ui.dark.toggleswitch.track.png" datatype="string" />
<parm name="ToggleSwitchThumbDisabled" value="res://UIXMobile!ui.dark.toggleswitch.thumb.disabled.png" datatype="string" />
<parm name="ToggleSwitchThumbOff" value="res://UIXMobile!ui.dark.toggleswitch.thumb.off.png" datatype="string" />
<parm name="ToggleSwitchThumbOn" value="res://UIXMobile!ui.dark.toggleswitch.thumb.on.png" datatype="string" />
<parm name="ToggleSwitchBackgroundDisabled" value="res://UIXMobile!ui.dark.toggleswitch.background.disabled.png" datatype="string" />
<parm name="ToggleSwitchBackgroundOff" value="res://UIXMobile!ui.dark.toggleswitch.background.off.png" datatype="string" />
<parm name="ToggleSwitchBackgroundOn" value="res://UIXMobile!ui.dark.toggleswitch.background.on.png" datatype="string" />
<parm name="ToggleSwitchTrackRight" value="res://UIXMobile!ui.dark.toggleswitch.trackright.png" datatype="string" />
<parm name="ToggleSwitchTrackLeft" value="res://UIXMobile!ui.dark.toggleswitch.trackleft.png" datatype="string" />
<parm name="DateTimeBackgroundSelected" value="res://UIXMobile!ui.dark.datetime.background.selected.png" datatype="string" />
<parm name="DateTimeBackground" value="res://UIXMobile!ui.dark.datetime.background.png" datatype="string" />
<parm name="ButtonBackgroundDisabled" value="res://UIXMobile!ui.dark.button.background.disabled.png" datatype="string" />
<parm name="ButtonBackgroundPressed" value="res://UIXMobile!ui.dark.button.background.down.png" datatype="string" />
<parm name="ButtonBackground" value="res://UIXMobile!ui.dark.button.background.png" datatype="string" />
<parm name="EditBoxBackgroundPressed" value="res://UIXMobile!ui.dark.editbox.background.down.png" datatype="string" />
<parm name="EditBoxBackground" value="res://UIXMobile!ui.dark.editbox.background.png" datatype="string" />
<parm name="RadioButtonBackgroundPressed" value="res://UIXMobile!ui.dark.radiobutton.background.down.png" datatype="string" />
<parm name="CommonErrorImage" value="res://UIXMobile!common.dark.error.png" datatype="string" />
<parm name="CheckBoxDisabledBackground" value="res://UIXMobile!ui.dark.checkbox.background.disabled.png" datatype="string" />
<parm name="CheckBoxBackgroundPressed" value="res://UIXMobile!ui.dark.checkbox.background.down.png" datatype="string" />
<parm name="CheckBoxBackground" value="res://UIXMobile!ui.dark.checkbox.background.png" datatype="string" />
<parm name="CheckBoxCheckmarkDisabled" value="res://UIXMobile!ui.dark.checkbox.checkmark.disabled.png" datatype="string" />
<parm name="CheckBoxCheckmark" value="res://UIXMobile!ui.dark.checkbox.checkmark.png" datatype="string" />
<parm name="TransparentBackgroundColor" value="#AA000000" datatype="string" />
<parm name="ListPickerBackgroundPressed" value="#FFDDDDDD" datatype="string" />
<parm name="ListPickerBackground" value="#FFAAAAAA" datatype="string" />
<parm name="ProgressBarDisabledBackground" value="#19999999" datatype="string" />
<parm name="ProgressBarBackground" value="#19FFFFFF" datatype="string" />
<parm name="SysTrayBackgroundColor" value="#FF000000" datatype="string" />
<parm name="ContrastColor" value="#FFFFFFFF" datatype="string" />
<parm name="ChromeColor" value="#FF0C0C0C" datatype="string" />
<parm name="PageBackgroundColor" value="#FF000000" datatype="string" />
<parm name="KeyPadBackground" value="#FF0C0C0C" datatype="string" />
<parm name="TextNeutralColor80" value="#CCFFFFFF" datatype="string" />
<parm name="TextNeutralColor75" value="#BFFFFFFF" datatype="string" />
<parm name="TextNeutralColor60" value="#99FFFFFF" datatype="string" />
<parm name="TextNeutralColor30" value="#66FFFFFF" datatype="string" />
<parm name="TextNeutralColor20" value="#33FFFFFF" datatype="string" />
<parm name="TextNeutralColor" value="#FFFFFFFF" datatype="string" />
<parm name="TextHighContrast" value="#FF000000" datatype="string" />
<parm name="BaseThemeColor" value="#FF000000" datatype="string" />
</characteristic>

<characteristic type="HKLM\ControlPanel\Themes\0\0C0A">
<parm name="DisplayName" value="Night" datatype="string" />
</characteristic>

<characteristic type="HKLM\ControlPanel\Themes\0\040C">
<parm name="DisplayName" value="Night" datatype="string" />
</characteristic>

<characteristic type="HKLM\ControlPanel\Themes\0\0407">
<parm name="DisplayName" value="Night" datatype="string" />
</characteristic>

<characteristic type="HKLM\ControlPanel\Themes\0\0409">
<parm name="DisplayName" value="Night" datatype="string" />
</characteristic>

<characteristic type="HKLM\ControlPanel\Themes\0\0410">
<parm name="DisplayName" value="Night" datatype="string" />
</characteristic>

</characteristic>

</wap-provisioningdoc>

Just put this provxml in your package and it will update the theme. If the light theme is already selected as the current theme you will need to set the theme to dark then restart and then it should be ok. Also if you have other languages included you will want to add them at the end or you can crash the theme system.