Update: The latest versions of Google Earth seem to be doing this by default now.
Google Earth has an official version for Linux and
it runs natively unlike Picasa, which uses WINE. But you wouldn’t know looking
at Google Earth’s interface in Linux; it looks like an ugly WINE application.
I’ve got a how-to if you haven’t yet installed Google Earth.
Google Earth uses the Qt toolkit, which is what the KDE environment is based on. But even in KDE, Google Earth will not take the system theme because its version of Qt is built-in instead of using system-wide library.
In Google Earth version 4.3, you can improve the appearance. Run Google Earth from the terminal, using this command for a GNOME look:
googleearth -style cleanlooks
And this for a KDE look:
googleearth -style plastique
You can update your menu item to launch Google Earth with your preferred
System->Preferences->Main Menu. Find Google Earth, right
click on it and select
Properties. Change the command for the item to one of
the two above.
This wouldn’t be necessary if Google provided a version which used the Qt library provided by your distribution.