Issues with Responsiveness and fixed header

Hi,
I have some issues with the theme, and I would be more than happy to get help ?? my site is https://www.media-buzz dot net/

Introduction: I can’t find the css code, it seems it isn’t concentrated in one file as in other themes, or I missed something?

Responsiveness:
1)I resized the size of the logo and the menu, and now when I the width of the browser is 1499px-1025px, the menu appears in two rows, although It has a lot of apace in order to remain in a single row.

2) The mobile menu start to appear in 1024px, which is too early. When I use 10 inch netbook or tablet in it’s wide posotion, there is mobile menu, and it isn’t good for me.

header:

I want the header to stay fixed. right now it is doing kind of jump when scrolling down, which is really annoying. (since I didn’t insert content yet, in order to see what I’m talking about, you need to reduce the height of the browser in order to see the jump when scrolling)

By thge way, this is the code I wrote in the css editor(if It can help..) thanks!

.logo.type-img {
padding-right: 15%;
padding-top: .65%;
width: 35%;
}

#nav-main {
float: left;
margin-left: 25%;
padding-top: 4%;
font-size: 160%;
color: black;
width: 35%;
}

#navigation-special {
position: absolute;
right: 710px;
bottom: 0;
}

#nav-main ul li {
padding-right: 2.5%;
padding-left: 2.5%;
padding-top: 4%;
padding-bottom: 4%;
border-radius: 20px 20px 0 0;
}

#nav-main ul li hover {
color: #990000;
}

#nav-main ul li.current_page_item {
background-color: #990000;
color: white;
}

#nav-main ul li.current_page_item hover {
color: white;
}

.header-wrapper.is-sticky .logo img {
max-height: none;
}

.header-wrapper {
border-bottom: 5px #990000 solid;
}