Html – Unable to replicate PyData Fonts

I’m trying to replicate the fonts used in PyData’s documentation. The custom CSS styling is taken directly from their website, however, my heading renders with a different font. PyData’s docs use Segoe UI Semibold, whereas mine renders simply as Segoe UI.
This is the custom styling added to the custom.css file.

html {
  /*****************************************************************************
  * Font features used in this theme
  */

  // base font size - applied at body/html level
  --pst-font-size-base: 1rem;

  // heading font sizes based on a medium contrast type scale
  // - see: https://github.com/Quansight-Labs/czi-scientific-python-mgmt/issues/97#issuecomment-2310531483
  --pst-font-size-h1: 2.625rem;
  --pst-font-size-h2: 2.125rem;
  --pst-font-size-h3: 1.75rem;
  --pst-font-size-h4: 1.5rem;
  --pst-font-size-h5: 1.25rem;
  --pst-font-size-h6: 1rem;

  // smaller than heading font sizes
  --pst-font-size-milli: 0.9rem;

  // Font weights
  --pst-font-weight-caption: 300;
  --pst-font-weight-heading: 600;

  // Font family
  // These are adapted from https://systemfontstack.com/ */
  --pst-font-family-base-system: -apple-system, "BlinkMacSystemFont", "Segoe UI",
    "Helvetica Neue", "Arial", sans-serif, "Apple Color Emoji", "Segoe UI Emoji",
    "Segoe UI Symbol";
  --pst-font-family-monospace-system: "SFMono-Regular", "Menlo", "Consolas",
    "Monaco", "Liberation Mono", "Lucida Console", monospace;
  --pst-font-family-base: var(--pst-font-family-base-system);
  --pst-font-family-heading: var(--pst-font-family-base-system);
  --pst-font-family-monospace: var(--pst-font-family-monospace-system);
}

$line-height-body: 1.65;
$fa-font-path: "vendor/fontawesome/webfonts/";

How would I go about replicating the font? I’ve tried changing the "font weight" parameter in the css file but it makes no difference.