Not that I would ever do this, but I'm just wondering hypothetically...say you have a dislike for a particular subject, for example group theory. Is it against the rules to downvote a question regarding group theory simply because you dislike the topic?
It is certainly discouraged.
However, we don't have many hard rules when it comes to voting; mostly just not to serially vote (up or down) a specific user. I don't think I can quite say it's against the rules, because I don't think any of the moderators would take any action against someone for such behavior (and a rule isn't really a rule unless there is some penalty for breaking it).
That being said, downvoting in such a manner in combination with other mean-spirited behavior could easily lead to a suspension for being "Abusive to others", one of our standard reasons for issuing suspensions.
That would not be well-received on the main site, although something like it could be standard on Meta.
On Main, downvoting is interpreted as "this is a bad/poorly researched/unclear/unconstructive question." On Meta, downvoting a question could mean "I don't think this question should have been raised," "I disagree with the conclusion the author is suggesting," or a host of other reasons, most context-dependent.