Thank you for your support. With code inspector I was able to identify the position in css where to change and my changes in codeinspector worked fine. But when I changed the coresponding css file, it had no effect on my website.
I changed the attributes in the following section:
#wp-advent-calendar li a,
#wp-advent-calendar li span {
border: 0;
color: #f4e536;
display: flex;
align-items: center;
justify-content: center;
font-size: 3em;
text-decoration: none;
transition: background 0.3s linear, text-shadow 0.3s linear;
height: 100%;
box-shadow: none;
}
