[Maerbenn]

There is a lot on the Themes in Morgoth's Ring. From Author's Note 1 to the Commentary on the Athrabeth:

The Eldar held that Eru was and is free at all stages. This freedom was shown in the Music by His introduction, after the arising of the discords of Melkor, of the two new themes, representing the coming of Elves and Men, which were not in His first communication.

The passage is annotated by Christopher Tolkien, and the note reads:

In the Ainulindalë (p. 11, §13) it was expressly stated that the Children of Ilúvatar ‘came with the Third Theme, and were not in the theme which Ilúvatar propounded at the beginning’. Of the Second Theme it is said in the Ainulindalë (p. 14, §24) that ‘Manwë ... was the chief instrument of the Second Theme that Ilúvatar had raised up against the discord of Melkor.’
 It is perhaps possible that by ‘the two new themes’ in the present passage my father was thinking of the introduction of Elves and Men into the Music as allied ‘themes’ that in the Ainulindalë were described as ‘the Third Theme’, but it seems to me more probable that a different conception of the Music had entered. In this connection, in a passage in the final rewriting and elaboration of QS Chapter 6 (p. 275, §50) it is told that Melkor spoke secretly to the Eldar in Aman concerning Men, although he knew little about them, ‘for engrossed with his own thought in the Music he had paid small heed to the Second Theme of Ilúvatar’. If this was not simply an inadvertence, it might support the view that the Second and Third Themes had become those that introduced Elves and Men - although it would surely be in the Second Theme that the Elves entered, and Men in the Third. It may be noted also that in the draft continuation of the letter to Rhona Beare of October 1958 (Letters no. 212), to which I have several times referred, my father wrote: ‘Their “themes” were introduced into the Music by the One, when the discords of Melkor arose’; and there is a further reference to ‘the Themes of the Children’ in Author's Note 7 (p. 342).