That's great approach. Yes you can add this in custom css file.
However, I have noticed that Q2A 1.7 version has changed class for custom menu item. Probably in next update I will add this in core.
I am adding your solution as a fix here so other can refer it.
Solution by proma
Add below code to custom.css file
.qa-nav-main-custom-21 .qa-nav-main-link {
background-image: url(images/nav-icons/custom-page.png);
background-position: right center;
background-repeat: no-repeat;
}
.qa-nav-main-custom-20 .qa-nav-main-link {
background-image: url(images/nav-icons/custom-page.png);
background-position: right center;
background-repeat: no-repeat;
}
.qa-nav-main-custom-19 .qa-nav-main-link {
background-image: url(images/nav-icons/custom-page.png);
background-position: right center;
background-repeat: no-repeat;
}