[MarkFL]

If you wish to use that classname in a CSS selector, you can either add the selector to your "additional.css" template, which will affect only affect the styles whose template you add the selector to, or you can use a plugin to add the selector to all pages regardless of style. Since the logo you want to change is likely only in one style, you should be okay to simply add the selector to your template:

.logo-image {
	width: 120px;
}