Test colours, backgrounds, fonts and formatting without affecting the theme files
Add your style properties to the boxes on the form.
Just go your website and you will see the updated styles.
Uncheck the 'Use the style editor' box and save. This won't lose your styles, it just ignores them when you view your site.
There could be loads of reasons. Your theme may not support this style, it may not be correctly formatted, your browser could be playing up, id and class names might be different. The plugin is just a testing platform, it's not a replacement for your theme stylesheet.
Yes. Just goto the plugin page and post a comment or send me a message.