I noticed that you can use a bootstrap plugin to add it to your site, but I have also read that github pages doesn’t support various plugins. I’m not sure if bootstrap is one of them.
So i’m wondering if there is some other framework that theme designers are using, or possibly something built into the core?
In all my themes I take a mobile first strategy when writing CSS.
Style for small screens first and use media queries to adjust things as the screen size increases.
You can absolutely use a framework like Bootstrap. It’s just CSS and not something you need a Jekyll plugin for.
Thanks this is a good reminder. I used to do a lot of css design from scratch, but then when responsive because necessary I started to cling to frameworks like Foundation. And then I gave up on front end for quite a few years.
I dislike introducing dependencies, so mine are build-in. And let’s face it, it is not as if it is difficult 
It often takes more time to get used to idiosyncrasies in ‘custom’ solutions than it takes to learn the principles oneself.
You are not force to use a plugin to make your website responsive. You can import Bootstrap SCSS files and design a Jekyll project with Bootstrap without limitations.