how to make header stay in position when scrolling?

how to make header stay in position when scrolling ? look at this screenshoot and this is my HUGOsite

i want to make my header stay or fixed when scrolling. i do know in wp but in HUGO i'm still learning. Please help me solve this. Thanks

trying to solve this header

Just add the following style on your pages, please get in touch with theme's author how to add custom style if you don't know how.

.header {
    position: sticky;
    top: 0;
    z-index: 1;
}

z-index is optional.