Responsive column card using flex

Your site is pretty good. Main thing's I'd add/modify are:

• adding a <main> tag to wrap around your page's main content. This is for accessbility purposes
• centering your container using flexbox. You can assign flexbox to your body. For example:

body{
   display: flex;
   flex-direction: column;
   justify-content: center;
   align-items: center;
}

• make sure you get rid of the margin: 100px auto from your container if you center it with flexbox
• you should also add some hover effects for your buttons. For example:

button:hover{
   background: transparent;
   color: white;
   border: white 2px solid;
}

• your container also doesn't change into column format when in mobile layout, this might be due to your using a max-width: 375px so when I tested with a 375px screen size it doesn't work as it needs to be less than it. My suggestion is to make the max-width around 420px. This way other mobile screen sizes are also supported.

Hope you find this feedback useful 👍