How to change the style of the section titles in a Quarto website sidebar?

I'm still struggling with setting up a Quarto website. In Quarto, the sidebar often has content. The styling of the sidebar, if I'm not mistaken, is inherited from the default themes. Imagining that I have a content with two levels:

Animals

• animal 1

Basics

• basic 1

Question: how to control the style of the first level headers, i.e. to make Animals, Basics and Humans bold, while leaving the rest of the formatting as it is.

Animals

• animal 1

Basics

• basic 1

Update

_quarto.yml

website:
  title: "Some fantastic site"
  page-navigation: true
  reader-mode: true
  bread-crumbs: false
  navbar:
    left:
      - href: index.qmd
        text: Home
      - about.qmd

 sidebar:
    style: "docked"
    search: true
    contents: auto

format:
    html:
      theme: cosmo
      css: styles.css
      toc: true

index.qmd in the main folder has nothing but the title. How to make header content elements bold in the sidebar?

You can target the first level headers and make them bold like this:

styles.css

.sidebar-item-text.sidebar-link.text-start {
    font-weight: bold;
}