THEME is a central idea. In nonfiction prose it may be thought of as the general topic of discussion, the subject of the discourse-- the thesis. In poetry, fiction and drama it is the abstract concept that is made concrete through representation in person, action, and image. No proper THEME is simply a subject or activity. Both THEME and thesis imply a subject and a predicate of some kind. For example, not vice in general but some such proposition as "Vice seems more interesting than virtue but turns out to be destructive." "Human wishes" is a topic or subject; the "vanity of human wishes" is a THEME.
When people say "there are no new stories," what they are really saying is that there are no new THEMES. Nearly everything-- every single THEME under the sun-- has been thought of, considered, written about and discussed in some context. It is how you write about your THEME that makes it something remarkable, not the theme itself.
(This post has been inspired by and in some instances, directly quoted from A Handbook to Literature, 8th Edition, by William Harmon and C. Hugh Holman)
"Never give up. And most importantly, be true to yourself. Write from your heart, in your own voice, and about what you believe in." ~ Louise Brown
"Write something to suit yourself and many people will like it; write something to suit everybody and scarcely anyone will care for it." ~Jesse Stuart
"A writer's job is to take one thing and make it stand for twenty." ~ Virginia Woolf