Upfront: How to set new (responsive) breakpoints?

Hi there!

On my site project http://oelziehen.net/ I have a "fixed" sidebar that is always visible.

Now while I hide that sidebar for smartphone, I would like to show that sidebar on tablets and medium sized screens.

The problem is, that with the current theme setting the sidebar "jumps" to the top of the page, when the screen size is smaller than 1080 px.

I would like the sidebar to stay there also when the screen is smaller.
Can anyone show me how this can be done? I granted access already.

Thx for your help! :slight_smile: