Five Template Custom CSS for Banner Button

Site URL: https://my-garden-guy.squarespace.com/

Hi there, hope someone can help. I have to add a CTA button to the homepage banner of a Five Template. I have added the following CSS but it is not working - it shows up as text above my banner image. Can anyone tell me what I am doing wrong? 

Thank you!!

#collection-5ecec952f66c3f2e64816014 .page-description a {
background: lightyellow;
padding: 20px 30px;
display: inline-block;
color: black;
border-radius: 50px;
}

Where did you add the code? Add to Home > Design > Custom CSS