You're currently setting the width of the footer to 100% but have a margin of 10px. By changing the width to: width: calc(100% - 20px) you can take the margin into account and eliminate the overflow.

By removing the justify-content: space-between; from the left menu, you can create a better look without having the unnecessary spacing between elements.

Currently the links don't differentiate which page the user is on. Remove the text decoration and add styling to show which page is active so the user knows where they are simply by looking at the menu.

Use the zoom attribute for the images as opposed to the current styling to allow it not to overflow the box and cause a jumping of text.

Make sure you're building reusable classes. For instance, you have the declaraton for flex-direction: column in a couple places where you should create a single class to help with those. You also have:

}
h3{
    text-shadow: -1px -1px 0 #000,
    1px -1px 0 #000,
    -1px 1px 0 #000,
    1px 1px 0 #000;
}
h2{
    text-shadow: -1px -1px 0 #000,
    1px -1px 0 #000,
    -1px 1px 0 #000,
    1px 1px 0 #000;
}
h1{
    text-shadow: -1px -1px 0 #000,
    1px -1px 0 #000,
    -1px 1px 0 #000,
    1px 1px 0 #000;
}

which since they're all the same declarations should be simplified to:

}
h1, h2, h3{
    text-shadow: -1px -1px 0 #000,
    1px -1px 0 #000,
    -1px 1px 0 #000,
    1px 1px 0 #000;
}

Adjust layout of contact form to reduce spacing. This could be as simple as moving the first and last names to the same row or adjusting the overall width of the form. Either way, adjusting the layout can reduce the negative space and create a better looking UI.