To apply folder-specific CSS styles that cascade down to all subfolders, you can place a custom.css file in any directory. The styles defined in this file will automatically apply to all markdown files within that directory and its subdirectories.
/* custom.css in the folder */
span.highlight {
background-color: #fff831;
padding: 0 4px;
border-radius: 3px;
font-weight: bold;
font-family: 'Courier New', Courier, monospace;
font-size: 2rem;
}