An Instagram Theme is the accumulation of images &videos which you upload to your instagram page which visually define your wall. Since instagram is all about having aesthetically looking images and high quality videos, niche topic imagery is the deciding factor when it comes to establishing a solid & loyal following as they drive engagement.
Having your own Instagram Theme, whether it is gardening, fitness, travel or food is fundamental as you lay off a base for your potential followers. Having a niche theme will let your followers know what the account is about and if they have the same interest, then they are more likely to engage. They will also know what to expect as each post comes in which drives engagement out of the roof, especially on Instagram.
Having an aesthetically looking portfolio of images is fundamental when it comes to first impression because the user might already like what you do before they even click on an image.