You can override theme styles in /assets/css/style.css or /assets/css/style.scss. Using Sass is especially helpful when you want to reference Jekyll variables.
You can add CSS to any page through the css_style variable in front matter.
---
css_style: |
.Link-btn {
background: rgba(0.1, 0.1, 0.1, 0.4);
color: #FFFF;
}
h1 {
color: #FFFF;
}
---