Template for a ArcGIS JS 4 mapping web app using calcite components
In index.html set the Calcite stylesheet url to the preferred theme
<!--link rel="stylesheet" href="https://js.arcgis.com/4.22/@arcgis/core/assets/esri/themes/light/main.css" /-->
<link rel="stylesheet" href="https://js.arcgis.com/4.22/@arcgis/core/assets/esri/themes/dark/main.css" />
And wrap application in a div
<div class="calcite-theme-dark"></div>
In main.css set the color scheme for dark themed browser controls e.g. scrollbars etc
:root {
color-scheme: dark; /* Dark style scrollbars etc.*/
}