I imagine a lot of people end up confusing the Theme for the Subject and, honestly…I did as well.
While I think the individual pages are decent enough for explaining the difference, I figured I might as well make this to make it a little clearer.

A theme is any abstract idea explored within a story, while a subject can be seen as what the story is about literally. To illustrate this example, here’s a mini-story:

The hatch from the Kar popped open, finally freshening up the cockpit with fresh air. Aubrey stayed seated in the cockpit, staring at nothing in particular, before pulling herself out of the Kar. She stumbled onto the catwalk and looked around her. The rest of the parts started emerging from their Kars, and the mechanics got to work. “Alright, that’s good enough for today. Head back to your sleeping quarters,” came the commanding voice over the intercom. Aubrey sighed and then started making her way to her quarters. On the way she bumped into a mechanic; he didn’t even glance back at her. Just before she entered the quarters, she noticed one of her fellow parts getting dragged off. One of the soldiers spots her looking, smiles and explains “This part is a little faulty, so we’re sending him off for some relaxation.” and then he walks off. She turns back to her door labelled “Part#38” – yup, that’s her. Aubrey enters her cabin and plops down on the bed. Time for the mandatory 7.5 hours of sleep before it’s back into the Kar.
The alarm starts to blare and she wakes with a start. She immediately gets up and rushes to the hanger. “Parts, return to the Kars to make them fully functional,” states the intercom. She jumps over the railing right into her Kar. The hatch closes and thus the cycle continues.

The subject of the story is mecha – it is literally about big robots being piloted by humans. The theme, however, is dehumanization, which is explored via showing how those pilots interact with those big robots.

Another way of looking at it is that themes can be emergent, while a subject cannot.