Editing the stylesheet in the plugin is not the correct way to do this. All of your changes will be lost after the plugin is next updated.
The recommended method for experienced developers to customise how the tabs display is to copy the css rules from the plugin’s stylesheet into the child theme’s stylesheet and then customise the colours and other CSS as required. When using customised version of the plugin’s styles in the child theme, you should also prevent the plugin’s default built-in styles from loading by adding the following line to the child theme’s functions.php (or a custom functionality plugin):
<?php remove_action('wp_print_styles', 'cc_tabby_css', 30); ?>
If you prefer to use a settings page in your WordPress admin to set a custom tab style, you can use the Tabby Responsive Tabs Customiser plugin which contains several tab style presets which can be further customised with a comprehensive set of easy to set options. The cusomiser plugin was designed to be easy for non-developers to use to customise how the tabs display.
