Sorry for late reply. Summer vacation.

You can get the default theme back by selecting 'default' from Conversations app's settings. Same place where you enabled the 'shaded'.
That changes opened conversations also.

If you have shaded theme selected, and uninstall the shaded package, the opened conversations don't update. But if you close them, they open with default theme the next time (or maybe the next after that :P).

(Though the messaging seems to keep the IM theme selection as 'shaded' still, if the theme isn't there, default is used. You can't see the selection if shaded was your only theme, though. But you notice this if you re-install shaded. It's selected in the settings again. And the second IM conversation you open after that will have the theme active.)

Default IM theme doesn't look the same as the SMS view. It doesn't have bubbles.