WooCommerce weird line on top of Checkout page

I’m using Woo Commerce with the Atahualpha theme.

I’m working on https://www.mamablessings.co.uk/checkout/
At the top of the checkout page you can see the area where you can add a discount code is formatted weirdly. The line is too thin and the info tab covers up some of the text, also the text is a weird colour and I’m not sure where that is coming from.

I am not a coder, I’ve googled and googled and come up with nothing at all.
Any ideas much appreciated.
Lisa