I think the sonnet 18 theme is showing Shakespeare's love towards his lover and that it will last forever even if the nature and other environment changes. Throughout the sonnet, I could really tell that he really loves her a lot.
yeah I agree with everyone ofcouse one of the major themes in this sonnet is Looooooove (I think it was towards women if Im not wrong?) also it was about how feelings never change even though time goes on (?) Im not very sure tho