Hello Lukofws,
Please add the following code in Appearance > Customize > Addtional CSS:
.error404 .button.button-404 {
background-color: tomato;
}
And for the hover color add this:
.error404 .button.button-404:hover {
background-color: red;
}
Replace the color with the ones you want and that’s it!
All the best,
Mihaela