Hi there,
Thanks for reaching out.
I understand that you would like to customize the payment form and apply some CSS on the form, and you are looking for the correct CSS selector to target that.
Would you please confirm if you want to customize the CC fields on the checkout form, as shown in the following screenshot below?
https://d.pr/i/d1V11g
If so, this form is coming from iFrame, and your CSS won’t be going to apply here. It is possible to customize those fields, but it requires some serious customization.
Custom coding is outside the scope of our support policy, but our developer resources portal can help get you going: https://developer.woocommerce.com/
You can also visit the WooCommerce Community Forum – https://ww.wp.xz.cn/support/plugin/woocommerce/ – the WooCommerce FB group – https://www.facebook.com/groups/advanced.woocommerce/ – or the #developers channel of our Community Slack – https://woocommerce.com/community-slack/ – We’re lucky to have a great community of open-source developers for WooCommerce, and many of our developers hang out there, too.
Take care, and have a fantastic weekend ahead.
Best,