Confession: I have always struggled with the word 'theme' even though I have an English degree and have written two novels! However, filling out the sentence shown below is a great way to get to grips with the theme of what you're talking about (and therefore needs to be expressed in different ways throughout your whole document).
I recently used this when writing an article about stormwater issues. "Nothing in STORMWATER (my subject) makes sense except in the light of LAND USE (my theme)."
You can also use it when figuring out what's at the core of your favourite TV series, and maybe why you gravitate towards it. Here are two of mine: