Sticky menu & change margins

Hello WordPress pros, I’m using the Tesseract theme and would like to make the header menu area sticky, so that when you scroll down the menu remains at the top of the screen. I’ve tried using plugins but those always come with animations or different formatting. I just want the existing menu to stay when scrolling. Is there a way to do this?

Also, I’m trying to make the margins narrower for the header, so that it stretches to both sides of the screen (like here: https://tyler.com). Any help would be appreciated. Thanks!

My site: https://qcsi.ca