Plugin is adding own styling for the features it adds, but there is no way to make styles completely immune to other stylesheets or styles added by other plugins or theme. Many themes are adding styles that are very aggressive and they can be made to have higher priority in styling certain elements. In that case, a theme can cause issues with the plugin features styling.
The only solution is to identify the styles that are causing problems and add new styles to fix that. This has to be done for every theme that causes problems.