Most plugins and themes don't need to do anything special to be multisite compatible. Themes especially. So just because a theme or plugin doesn’t say it’s Multisite compatible doesn’t mean it isn’t.
Sometimes plugins don't behave well if you Network Activate them, but the workaround for that is often just to activate them on individual sites instead, which will generally avoid most issues.
If you have a specific issue with a theme or plugin, there isn't a single solution that can be applied to all themes and plugins that will fix multisite compatibility issues, so you will need to take it up with the plugin author.