UberMenu loads two CSS stylesheets by default: the core ubermenu.min.css file and a skin file of your choosing.
Please note that not all plugins’ assets can be properly optimized. If you have another plugin that causes problems, it can break UberMenu when all the CSS and JS is combined. In this case, you’ll just need to disable the optimization plugin, or configure it to avoid errors.
Note that you should not enable caching or optimization plugins during development, as they can prevent you from seeing changes to your site