"I do not like rice."
Ok, so moving forward I found out that the reason is that verbs that are actually adjectives such as 好き ask for が. Thought I would leave this here for future people. Now for the folks asking は vs が this is what I've gathered so far:
は: Used to indicate the topic, what it is that we are talking about; used to introduce a new topic in an ongoing coversation; used in negative and interrogative sentences; can be used along with に if the place is also the topic.
が: Used to identify a subject among multiple options, bringing it to the one in the conversation (what I first thought was the case here); used to indicate the topic when it had been presented before; used to indicate objects that are described by adjectives (what actually is the case here).
I think we would't use it as much on an actual conversation because the subject is usually ommited. Like here, imagine someone asked you what you think of rice, you could answer with きらいです and they would know you're talking about the rice. Something you learn here is that in japanese less is always more and they only say the essencial.
But yeah, if you're on the same topic, say, John, the second time John is mentioned he will be a が. If you bring Mary to the talk she would be a new は (unless she's a も which you'll see soon)