Paul

You may choose a colourful style for the custom coupon message, or wrap the message in your own CSS class container.
It’s also possible to hide the default message from Woocommerce so that only your custom coupon message is displayed. 
(the plugin uses inline CSS to hide the default Woocommerce message).
Take a look at the screenshots for the coupon edit screen with the plugin enabled and some examples.
The meta box was implemented using the technique described on this blog post: 
http://wp.smashingmagazine.com/2011/10/04/create-custom-post-meta-boxes-wordpress/
The message styles were generated using this tool: 
http://www.bestcssbuttongenerator.com/

Woocommerce Custom Coupon Message

This plugin adds a meta box to the coupon edit screen where you can make your own customized coupon message.

RLDD

Apply WooCommerce Coupons automatically with a simple, fast and lightweight plugin.
<h3>Features</h3>
☑ Unlimited Auto Apply Coupons using native WooCommerce coupon conditions
☑ Set Minimum Product Quantities for coupons
☑ Apply Coupons by URL when a specific page is visited
☑ Coupon Troubleshooting by itemized coupon on the cart page
☑ WooCommerce Blocks Compatible
<h3>PRO Features</h3>
✅ Auto-Apply Defaults – Default future coupons to auto-apply automatically.
✅ Bulk Updates – Update auto-apply on active coupons in bulk.
✅ Copy Coupons & More – Duplicate coupons, products, posts and pages.
✅ Streamline Email Restrictions – Prompt for customer email in cart when a coupon with email restrictions is applied.
✅ Coupon Defaults – Set default verbiage for new coupons.

Auto Coupons for WooCommerce

Apply WooCommerce Coupons automatically with a fast, lightweight plugin. Set minimum product quantities, apply coupons by URL or automatically.

Woocommerce Custom Coupon Message vs Auto Coupons for WooCommerce