The most common and most likely answer to this crossword clue is the 8 letter word KEYNOTES
I've seen this crossword clue in The Sunday Telegraph
K
E
Y
N
O
T
E
S
Buddy explains!

The straight part
We know that "Main themes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main themes = keynotes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does KEYNOTES mean?
KEYNOTES noun- The note on which a musical key is based; the tonic.
- The main theme of a speech, a written work, or a conference.
- A speech that sets the main theme of a conference or other gathering; a keynote speech or keynote address.