Customize the plugin css

If you want to customize the default styling included with the plugin, you easily do so in an upgrade-safe manner by copying the default CSS file to a specific folder inside of your theme, and then modifying the file to fit your needs.

Create a folder inside of your theme called wpum, if it doesn’t already exist.
Copy the file from wp-content/plugins/wp-user-manager/assets/css/wp_user_manager_frontend.css to the new wpum directory.
Finally, the plugin will load your custom version of the CSS, instead of the one provided by the plugin.

If you have a caching plugin - clear the cache.