What are the key themes in Ted Hughes's poem, "The Thought-Fox"?

"The Thought Fox" is an example of what's called metafiction. This simply means a piece of writing which is about the process of writing. So Ted Hughes has written a poem about Ted Hughes writing a poem. He's sitting all alone at his desk in front of a blank page. The blankness of the page is matched by the blank, featureless landscape outside the window. The sky is starless; the world outside is silent and black.
The blank page tells us that the poet's inspiration has run dry. But all of a sudden he becomes aware of a presence outside; his imagination begins to stir:

Something more nearThough deeper within darknessIs entering the loneliness.

That something he sees is a fox:

Cold, delicately as the dark snowA fox's nose touches twig, leaf;Two eyes serve a movement, that nowAnd again now, and now, and nowSets neat prints into the snowBetween trees, and warily a lameShadow lags by stump and in hollowOf a body that is bold to come . . .

The fox comes closer, at once wary and bold. It's so close now that its eyes have merged imperceptibly into a single green glare. And then . . . 

Till, with a sudden sharp hot stink of fox,It enters the dark hole of the head.The window is starless still; the clock ticks,The page is printed.

So, it wasn't really a fox after all. It was a creation of the poet's imagination. The world outside remains silent, black and still; the clock ticks on, same as always. But crucially, something has changed: the piece of paper in front of the poet has been transformed. Now it's full; the poet has finally written his poem. And his imagination has been rekindled as if by magic, allowing him to create something out of nothing. The process of artistic creation presented to us by Hughes is almost God-like in its scope and intensity. The main theme is creative inspiration; just as God can create anything at will, so too can a great poet endowed with inspiration: his "fox," if you like.