Although this might be present in other versions of DNN too, I've found that if I add the attribute "theme=something" to the <pages> element in web.config, I get a compilation error in FCKImageGallery.aspx:
"Error 129 Error parsing attribute 'theme': The 'theme' property is read-only and cannot be set. "
That is because the page's codebehind class derives from the FCKGallery class, which already has a Theme property. This is unfortunate, because, although DNN has its own skinning mechanisms, you might also want to use the standard asp.net themes (like for example for CSS Friendly Control Adapters).
The only workaround I've found so far for this is to add a web.config file to the Fck folder that looks like so:
<configuration>
<appSettings/>
<connectionStrings/>
<system.web>
<pages theme=""> </pages>
</system.web>
</configuration>
This "local" config file basically overwrites the application web.config file and tells the compiler that themes are not needed for the local Fck folder.
As I'm new to DNN, maybe I'm missing something, so if you know anything about this, please let me know ;)