CSS for expanding send button

See Styling Contact Form – there is a link to an article I wrote at the bottom of that page (which is recommended by the CF7 plugin author, though I’m not allowed to link directly to it here), that covers this topic in detail.

Abbreviated Version

1. You need basic HTML & CSS skills
2. You need to know how to use Chrome Dev Tools or equivalent
3. Use Dev Tools to examine the CSS styling applied to your form in detail
4. Determine what CSS changes you need to make
5. Add custom CSS via WP Customizer > Custom CSS, custom CSS plugin or child theme.