Remove Footer from Lightning Action Visualforce

To hide the header, add showQuickActionVfHeader=“false” to the apex:page>

Document Link : Hide the Action Header for Visualforce Custom Actions

Example :

<apex:page standardcontroller="Account" extensions="AccountController" showQuickActionVfHeader="false" >

Thanks

Anup

You will not be able to impact the styles of the action modal from your Visualforce because Salesforce uses an iframe to place your content. This is a security measure and is explicitly meant to stop you from stepping outside the bounds of your page (even if you are trying to do something harmless like hide the modal buttons).

If you are willing to convert your page to a lightning component take a look at the force:lightningQuickActionWithoutHeader interface which will allow you to make a component quick action without the header or controls.

Additional details can be found here.

Just to give a preview this is what an example looks like: