Ok, so I started out just like you and knew absolutely nothing about Javascript or app development or anything and now my company uses an app that i coded from scratch with a custom theme and everything. I'm not tooting my own horn I'm just saying that I know exactly where you're coming from and I could be a good resource for you! Plus I'm an Aggie myself (class of '13). So here's where I started:
This is tutorial is a decent resource to getting started with making your own theme:
Create a theme—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers
There's a decent ESRI training video that shows you how to create your own controller widget that i'm trying to find for you, but having trouble locating it. I'll keep looking.
I also followed this tutorial to create my first widget and from there began experimenting with it and customizing it.
Create a ListView widget—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers
Do you have experience with Javascript and HTML? I bought a couple textbooks and read through them to start learning how to code using those two languages. I bought these books:
https://www.amazon.com/Web-Design-HTML-JavaScript-jQuery/dp/1118907442/ref=sr_1_4?ie=UTF8&qid=152632...
Let me know what you're next questions are! I'm looking forward to helping you out!