Custom div class in Quarto beamer output

I would like to know if it's possible to create a custom div class in a Quarto document with a beamer output format. I have tried with adding a custom environment with include-in-header: preamble.tex, and using ::: {.name_of_environment}, but it does not work. I expected a similar behavior as when one creates a custom class in css for Quarto html files.

Here's a minimal example. I know I can use shrink here, but this question applies to other situations as well, where creating a new class/environment in a Quarto beamer document would prove useful.

test.qmd:

---
format:
  beamer:
    include-in-header: preamble.tex
    theme: metropolis
---

## Test slide

<!--- This is too big. I would like the slide to shrink -->

This is a test slide with some content. It has a few points:

- Point one
- Point two
- Point three
- Point four
- Point five
- Point six
- Point seven
- Point eight

And some more content to fill the slide.

The slide also has a note:

::: {.callout-note appearance="minimal"}
This is a note that provides additional information about the test slide.
:::

## Test slide {.customshrink}

<!--- This is too big. I would like the slide to shrink using a custom shrink function defined in preamble.tex -->

This is a test slide with some content. It has a few points:

- Point one
- Point two
- Point three
- Point four
- Point five
- Point six
- Point seven
- Point eight

And some more content to fill the slide.

The slide also has a note:

::: {.callout-note appearance="minimal"}
This is a note that provides additional information about the test slide.
:::

preamble.tex:

\newenvironment{customshrink}
  {
    \scriptsize%
    \setlength{\parskip}{0pt}%
    \setlength{\itemsep}{1pt}%
    \setlength{\topsep}{0pt}%
  }
  {
    % nothing needed here
  }