In general, the one thing you should not do is edit the theme files directly. If the theme is ever upgraded, you will lose your changes. The suggested practice is to create a Child Theme and make any changes there. If you don’t feel that you will ever install upgrades or patches to your theme, or if your modifications are minor and easy to reproduce (and remember) in case of a theme upgrade, then editing the theme files should be OK (but is not recommended).
That being said, if you are just making CSS changes, you don’t really have to create a child theme. Sometimes a theme has an option to add custom CSS, and that’s where you should make your changes. If your theme does not have a custom CSS option, then you can install a plugin that will allow you to add custom CSS, like Lazyest Stylesheet or Custom CSS Manager. Any custom CSS that you add will override the theme’s CSS, since the custom CSS will appear after the theme’s style.css file. So adding a rule like this to your custom CSS:
.gallery-item .gallery-icon {
background-image : none;
}
Will override the theme’s setting for that same selector. Note that I used the class names that are specific to the gallery elements instead of the element types, just in case there are other places where the dl dt elements are used outside of the gallery context.