Two words will keep coming up, so let me define them cleanly. Your niche is the subject of your page, the topic you cover. Your theme is the identity of your page, the look and feel that makes a post recognizable as yours. A strong page has both. The niche gives people a reason to follow. The theme is the reason they remember you.
When you weigh a niche, run it through three simple questions. Does it attract Tier 1 advertiser money? Is there an endless supply of content you can pull from, so you never run dry? And can you stand making it every single day for a year? A niche that passes all three is a niche you can grow.