What kind of extracurricular should I do, which is somehow connected with theater art/forensic science/criminal justice?

There are tons of opportunities and extracurriculars that are possible but it depends on what you choose to do. For theater arts, you could participate in musical productions in your high school or community. Your high school might offered some technical arts, drama, or theater production classes that would be really helpful. Your teachers in those classes would definitely know and provide more opportunities on the subject.

For forensic science, there are sub competitions in many clubs like Technology Student Association (TSA) that have categories on forensic science and criminal justice.

again, because these might be classes that are offered in your school, the teachers would 100% know more and have more info. Some clubs to think about are debate team. because I think that that would connect well to your interests.

Other opportunities like shadowing people in those professions would help you decide whether or not that's what you actually want to do. I don't know how internships would work for forensic science and criminal justice but I am sure that there are opportunities out there. Just look it up and you might find some. I think it will be harder to do this remotely because of how hands on some of the topics can be.

I hope this can help you!

With travel and face-to-face opportunities still a bit spotty, I'd suggest going local for both. Ask if you can volunteer at your local theater/performance venues, and volunteer to do anything to get your foot in the door to start networking. Building sets, janitorial, snack bar, ticket sales will show them what kind of worker you are and give you access to productions as they start rehearsing again. They are much more apt to give you a supporting role after seeing your (strong) work ethic in action. For forensic sci/cj, begin with your local law enforcement's and district attorney's front desk (preferred to over the phone since it is easy to say, "Nothing right now" over the phone, but easier for you to be more persistent in person) to see if there is a way to volunteer for them, programs for HS students, advice on where to begin. Good success!

For forensic science/criminal justice, the FBI offers a junior internship program for high school students 16 and under. You should get a teacher to recommend you to strengthen your application. Check and see if your FBI field office offers the program. On the FBI's website, they have a page for high school students and what you can do to prepare for a career there if that's something you're looking for. As for theatre, you might consider investing in a dance class such as jazz or ballet. This can help you get more or better parts by enhancing your experience in dance, especially if you do musical theatre. Personally, I started dance classes at 15, which is typically considered a late start, and now I'm 17 and I love to dance and have better breath control in my singing. If your school has a dance class or team, consider joining or trying out. You don't have to compete or perform in dance if that's not something you're looking for, but dance classes have a lot of benefits from flexibility in your body and your mind, to better strength in your legs and core, to artistic expression as an outlet. You can also take virtual masterclasses for acting, singing, and dancing. I hope this helped and good luck with your search!