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Can you tell me how to add actions and categories?

The easiest way is to use either phpMyAdmin or mySQL GUI tools to directly edit the database. You should look for tables called wp_actionable and wp_actionable_categories (wp_ is the default WordPress database prefix - if you have changed this look for the table names with your prefix.) Create some categories in _actionable_categories first then create some actions in _actionable_actions assigning actionable_id in _actionable_categories to actionable_cat in _actionable_actions for the appropriate action.

The next release will allow for category and action creation from the admin panel

Can I add my own styles?

Yes, just edit the file wp-content/plugins/actionable/css/styles.css.