What are the key themes in Ted Hughes's "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 near
Though deeper within darkness
Is entering the loneliness.

That something he sees is a fox:

Cold, delicately as the dark snow
A fox's nose touches twig, leaf;
Two eyes serve a movement, that now
And again now, and now, and now

Sets neat prints into the snow
Between trees, and warily a lame
Shadow lags by stump and in hollow
Of 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. 

What quotes in Ted Hughes' "The Thought Fox" reflect tenderness and violence?

In the first few stanzas, the fox is presented as a gentle creature, warily finding its way through the snow. As a wild creature, the fox presents no immediate danger to anyone nor is it being hunted. The fox is not aggressive as illustrated in the following:

 "Cold, delicately as the dark snow,
  A fox's nose touches twig, leaf"

"Delicately" suggests a tenderness and the inverted simile suggests the fragility of the fox: it is as fragile and gentle as snow. The image is further emphasized by the words, "nose," "twig," and "leaf" -- these are all delicate objects. The nose is a sensitive organ and a twig and a leaf are vulnerable and can easily be damaged or broken. 

"Two eyes serve a movement, that now
 And again now, and now, and now
 Sets neat prints into the snow
 Between trees, and warily a lame
 Shadow lags by stump and in hollow"

These lines further accentuate the mild character of the fox. It is timid and unsure, suggested by the repetition of "and now" which describes its tenuous movements forward. It is wary and feeble and hangs back, unsure of itself. All this is symbolic of the gentle nature of this creature.

"a body that is bold to come"

in line 16 indicates that the fox's body language suggests a desire to be less careful, less timid. The fox seeks to express itself and be defined.

The next few lines build the suspense as the fox comes ever closer and one can discern its finer details, which grow larger. The fox is:

"Coming about its own business"(line 20)

It is there for a purpose of its own. This suggests an almost invasive presence, intent on achieving a specific purpose. The erstwhile uncertainty has gradually faded.

"Till, with sudden sharp hot stink of fox
 It enters the dark hole of the head."

The former gentle timidity of the fox explodes into a violent expression of its presence. Ted Hughes emphasizes this suddenness through the alliteration of the 's'. Its presence is pervasive. The fox has violently entered its lair (the dark hole) and has announced its presence by sending out its "sharp, hot stink." These images are in sharp contrast to the previous gentle terminology. The words suggest an intensely forceful presence.

Ted Hughes has brilliantly used the arrival of a metaphoric fox to symbolize the genesis of an idea, its gentle growth culminating in its sudden, almost violent, expression.