#header #navigation edit

are you using a child theme or trying to directly edit the parent theme CSS?

in some themes the CSS for a navigation menu is controlled with media queries in the CSS file.

this means that the same media query needs to be used when you modify the border, colors etc of the navigation menu too.

are you familiar with CSS and media queries? if yes, I can provide more direction to help with the changes.

Maybe the problem is that I am just familiar with CSS:) Please, try to help me, maybe I will manage do it. And I don’t know what is media queries

media queries is a part of CSS.

if you want to learn how to modify and manage css for your website here are a few resources to start with:

https://css-tricks.com/
https://www.w3.org/Style/CSS/
https://www.codecademy.com/tracks/web

Here is a pretty good easy to follow tutorial to get your started.
https://thethemefoundry.com/blog/how-to-customize-a-wordpress-theme/

thank you a lot