How to make a div fill a remaining horizontal space?

These days, you should use the flexbox method (may be adapted to all browsers with a browser prefix).

.container {
    display: flex;
}

.left {
    width: 180px;
}

.right {
    flex-grow: 1;
}

More info: https://css-tricks.com/snippets/css/a-guide-to-flexbox/