The code depends on what you want to achieve, for example, adding this code
.blog-post .post-categories {
background-color: green;
color: black;
}
.blog-post .post-categories a {
color: black;
font-size: 30px;
}
will give – http://prntscr.com/oqfx0z
We can help you with the custom css code, just describe what exactly do you want to change (font-size, border, background?).
https://www.w3schools.com/css/
Regards