Someone so kind to explain how to make a theme responsive?

[WallBB]

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