I can't guarantee your friend will understand your reasons for not wanting to come, you know her best and are, as always, the best judge of her possible reactions to telling her the truth. But there are quite a few things you already did right in your question to make your message as non-confrontational as possible:
- Using an I-message. You wrote your question to explain about you, and your discomfort, instead of saying your friend isn't nice for throwing a rival themed party and she should've known better!
- You've given an honest reason, and asserted your feelings on the matter. Though this may kinda be caused by EmC's edit too... Assertiveness is being confident about your own feelings and reasons, but without making the other person feel threatened in their rights. Rhetorical questions like the one originally in your question probably aren't the way to go if you don't want your friend to think of you as confrontational.
There's another thing you could consider:
Compromise. Find out what you and your friend both value, and work towards achieving that. Because if you go in with only what you value, a compromise might feel like a bad deal for your friend:
Research has indicated that suboptimal compromises are often the result of negotiators failing to realize when they have interests that are completely compatible with those of the other party and settle for suboptimal agreements. Mutually better outcomes can often be found by careful investigation of both parties' interests, especially if done early in negotiations.
If both you and your friend value e.g. your presence more than the themed party being perfectly themed, a compromise might work. If your friend values a themed party more than everyone being present, pushing for a compromise where you're present as well will likely make her feel bad, no matter what. I suggest you keep this option open as well, instead of insisting you won't be there immediately, you could open a conversation about your feelings on the party and try to find out whether your friend is open to make some accommodations for them, like not having you wear rival club colours or not taking pictures. Or even just having you around for tea another time.
In my experience, a combination of these three works well for me when declining invites to parties I don't feel comfortable going to (like ones where I know there will be a lot of alcohol/drinking games, drugs, and people that I don't get along with).
I'm mostly honest about my reasons for not wanting to go, as this sends a signal that I'm not really interested in these types of parties. Some 'courtesy' invites can't be avoided, but they are very easily declined and the declination often accepted without any objections. I must admit I am more likely to just decline with a 'sorry, can't make the date/time' if I'm not that close to the person inviting me.
With one or two close friends, I compromise. It either ends up with me being there early, doing some party prepping, talking, then leaving early at the official party to avoid the 'worst' of that. Or (more often) having a different moment where we go out for food/tea/coffee.
