Basic template needed

I've been developing up some online course content in a couple of niche fields, but have really struggled to get hold of a basic template for my company that looks polished, cohesive, engaging & recognizable. My courses will have relatively few images (I understand it's unusual, but more diagrams than most too). Something in 2-3 key colours with multiple layouts that help to orient a learner but also look coherent. Why is this so incredibly difficult? Is there somewhere I can buy a base set of decent slides outright, or a base I can tweak to suit myself? I'm completely stuck until this can be solved.