Music Theory is a valuable asset to any musician and can help your playing and sight reading improve a great deal.
Plus, there are teach yourself Online theory courses now available...
This website offers a simple introduction to music theory. It's not based around any theory exams but gives a good grounding on the basics. Following a few pages as a 'taster' you can buy a course for as little as US$29 at the moment, which is pretty good value.
This is a fantastic website with lots of free resources. It has links to videos and quizzes and is a great place to find help at the Grade level you want.
With ABRSM you can sit your theory exams online. There are various rules you need to abide by but it's a much more convenient way to sit the exam, particularly if you are ready to progress to Grade 6 and you need the Grade 5 theory result before hand! These exams used to be held 3 times a year in a large hall, so I'm glad this is now being offered instead.
For further info go to: https://gb.abrsm.org/en/our-exams/music-theory-exams/