Customise "was added to your cart" message

Add the following to Website > Pages > Website Tools > Custom CSS.

/* begin cart item added popover */

  .template-cart-item-added-popover {
  
    background-color : #C7C7C7;
    border-radius : 15px;
    text-transform : lowercase;
    
    }
  
  .template-cart-item-added-popover p {
  
    color : black;
    
    }
    
  /* end cart item added popover */

This is for v7.1.

CSS can not be used to change the period to exclamation. For this you would need JavaScript and to use a Mutation Observer. I know of no code that currently does this.