You can't use 2 themes at the same time BUT you can include one theme file in another using the include tag. This is done in the ThemeEditor's gui.xml file.
Now if you want to overlay different UIs then you need to define how these should react to events. It is easy to use something like this:
Root Widget (a normal Widget)
| |- ResizabelFrame
With "freeze" you mean that they no longer respond to mouse/keyboard input? This is mostly a sign that they are either below a (transparent) widget which intercepts mouse events or that their size does not match their visual appearance. To check for the later you can enable setClip(true) on these widgets or their parents.