Usual:

format: 
  revealjs:
    logo: logo.png
    theme: [default, custom.scss]

Adding after the first one, so no !important needed¹:

format: 
  revealjs:
    logo: logo.png
    header-includes: |
      <link href="custom.css" rel="stylesheet">      

Inline w/o declaring classes²:

Some [red words]{style="color:#cc0000"}

or:

::: {style="font-size: 1.5em; text-align: center"}
styling an entire div
:::

• Main Quarto Reveal SCSS: quarto-cli/src/resources/formats/revealjs/reveal/css/reveal.scss at 303c4bc9f52ea0b0c36c5eb5dfbade9cece2b100 · quarto-dev/quarto-cli
• Quarto footer css: quarto-cli/src/resources/formats/revealjs/plugins/support/footer.css at main · quarto-dev/quarto-cli