Hello,
When you use the Category menu in the header, you cannot change its width. If you show the Category menu in the content, by means of Mega menu widget element you can change the width by means of column width and set the color on the column https://xtemos.com/docs/woodmart/adding-menu-site/
Please add this code to the Theme Settings > Custom CSS > Global:
.vertical-navigation .item-level-0.menu-item-has-children>a {
background:red;
color:green;
}
body .vertical-navigation .item-level-0:hover>a {
background-color: green;
color: red;
}
Set the colors as per your needs.
Best Regards