A good thesis for an essay on theme will examine the importance of a theme in a text. A theme is the central idea in a literary work. Therefore, a good thesis statement will set up a critical examination of the importance of themes in literary works.
A good thesis statement will set the focus for the essay and, therefore, tell the reader what the essay is going to be about.
Below are examples of thesis statements regarding the importance of themes in literary works.
-- The theme of any given literary work allows the text to show coherence in regards to the central idea delivered in the text.
If the essay is focusing upon a particular text, the thesis statement regarding the theme could look something like this:
-- The development of the theme in _____________ is important given it is illuminated throughout the entire text and forces readers to ____________________.
-- Themes are important in any text given they allow a reader to focus on both the direct and indirect messages provided by the author. This focus allows a reader to find coherence between what is being implied and what is being stated.
On top of providing suggestions for a thesis statement, I have provided a link below which will walk you through the process of creating a thesis statement (as provided by eNotes).