The styling of this website is happening in a number of different places. some of the highlevel changes are set in the format section of _quarto.yml, with the rest of the main styles set in styles.scss.
The front page includes a number of detailed styling, these are all located in styles-frontpage.scss. They are all wrapped in #FrontPage ID so they shouldn't affect anything not located in the front page.
The sidebar for the Get Started section has a unique style, and that is specified in the start/styles.css file, that is loaded into each of these pages with either css: styles.css or css: ../styles.css.