title: Can I detect if a specific CSS custom variable exists in the loaded styles?
body: <p>I am creating some custom web components. One of the components is a light/dark theme switcher. However, this relies on a specific stylesheet being loaded.</p>
  <p>I would like to output a warning if the correct stylesheet has not been loaded.</p>
  <p>But, users have several different ways of loading stylesheets (e.g. link tag, @import statement, etc) and so it is not guaranteed that they will have loaded the correct sheet.</p>
  <p>The stylesheet that is needed has some pretty specific custom variables and named styles in it. So I would like to know <strong>if there is any way from JavaScript (inside my web component), to check whether one of those variables or style names exists in the stylesheets loaded to the current page</strong>.</p>