Fixing the sidebar width

Good day WordPressers out there,

I am trying to set a fixed width for the sidebar on my WP site. Currently the width of the sidebar changes in three levels as the browser width changes until it switches to the mobile view. I’m trying to fix the sidebar width without disabling the responsive mode. Everything on the site is fine adjusting to the browser width change except the sidebar width, until it turns in to the mobile view.
This seems to be set by the theme that I’m using and I’ve looked into CSS Media Queries instructions, but I cannot figure it out..
Any advise is appreciated.
Best,