all of them! I know it seems daunting, but a lot of the stuff is transferable across themes, like for old texts stuff from religion can transfer over to stuff on love/marriage.
In general there are more than just 4 themes, i did inspector calls at GCSE, and here are the themes i can recall... Class, Capitalism vs socialism, gender, marriage, setting (setting questions are always stupid, but they do come up,)Power, age, social responibility etc.... but find about 20 really good quotes that are very transferable and just learn them. unfortunatley at gcse there is really no way of getting around learning quotes.
My tip is, make a template for a paragraph, and then just slot in the stuff relating to whatever the theme is. If you are really struggling, then if you look at past papers, you can see what themes they havent done yet, those are more likely to come up (but others could still come up!)