I've made a few amendments to the "Blue" theme's blue.css file to better match the forum to the overall site styling.   The "Forum" page that the forum module is added to and the forum itself looks exectly how I intended it to.

However, most of the forum related pages linked off the forum (My settings, Reply, Quote, Edit, New Thread etc) revert to the default Minimal Extropy skin even though the "Forum" page, the portal and the host settings all have the new skin selected and associated containers selected.  The My Threads and My Posts pages behave properly though. It seems to only be an issue for pages which have /ctl/ in their path (eg http//www.[domain name].com/Forum/tabid/55/ctl/PostEdit/forumid/11/postid/2899/action/quote/mid/367/Default.aspx).

For an example of what I'm getting go to the forum at http://www.fenderavenue.com/Forum/tabid/55/Default.aspx and click on the "search" button.

Can anyone tell me which pages determine the appearance for the forum associated pages such as My settings, Reply, Quote New Thread etc (ie those with /ctl/ in their path)?  

I'me using DNN 3.5.1 and Forum module 4.0.0.1

Any suggestions would be greatly appreciated.
cheers, Rob B

Many thanks for the quick response Richard.  All makes sense now and is working as it should be. While I've administered websites before I haven' t had any experience in the DNN environment - but I'm getting the hang of it pretty quickly.
Much appreciated
Rob B
ps.  the forum module version is 5.00.01 - not as I had stated in my OP.