This is happening because of the custom code you added to your css. When you add a fixed width it will always be that size unless you add the code in your responsive section of the css to override the width when a smaller/mobile browser is used.
It looks like you added a width to body.custom-background in your custom css –
body.custom-background {
background-color: #333;
width: 1140px;
margin: 0 auto;
}
There is also a width added to the .site-inner in your custom css.
Last is the .content where you added 779px –
.content {
width: 779px !important;
padding: 0 25px;
}
Once you remove or adjust these back to the original size your responsive code will work again.
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