How to target a Div in CSS in Rmarkdown HTML

Question

I'd like to be able to control the header of this HTML file with CSS inside Rmarkdown. I want to change the text size, but can't seem to control it, here is what I have tried

div.header.fluid-row {
  font-size: 12;
}

div.fluid-row {
  font-size: 12;
}

Answer 1

Maybe so?

.fluid-row .title {
  font-size: 12px;
}

/*    OR    */

.fluid-row h1 {
  font-size: 12px;
}