MyBB Community Forums

It could be useful for a lot of people, other than me. 

There's no simple way to make a theme responsive other than knowing CSS. Media queries are one of the most common features used, although they turn out to be quite weak if used alone or without proper HTML markup. I'd suggest you to use a framework with pre-built responsive elements, such as Bootstrap. It's almost impossible for a single developer to keep up with all the necessary edits needed if you're not using a framework, which should also step up your theme game by several folds as they usually already come with modern elements and different variants you can apply with relative ease.

Shade has given a good answer, I will give you another aspect in terms of learning curve. if you are familiar with designing responsive templates, you can start getting involved in theme designing yourself. If you are not then start reading tutorials here on MyBB.
For example, I wrote few tutorials in past, one of them is to how to make header menu responsive (https://wallbb.co.uk/simple-three-step-t...bb-forums/)

The above tutorials gives you a grasp on how templates control the aspects of theme and how you can use css to design and give your own queries.

Regards
WallBB

Thank you guys.