I'm working with Orchard 1.7.2 and in my custom module I have a number of controller actions decorated with the [Themed] attribute.
My problem is that when I also decorate any of them with the [HttpPost] attribute the theme stops working when that action is used.
Has anyone come across this before?
Sure, you're telling the action that it shouldn't run for get requests, just for post request. An action marked as post is used for ajax request or for sending a form to the server. (As you point is legitimate to do a post request for a page, I deleted that part, but again is not recommendable)
Try to inject a IWorkContextAccessor, RequestContext and a IThemeManager to your constructor, then in your action do something like this:
I don't know if that works, but at least you have a start point.