server/boot/app-functions.js
function getInheritTheme(theme, tree = {}) {
  if (!app.locals.themes[theme]) {
    // this theme is not installed
    app.error('requested theme:%s but it is not installed', theme);
    return tree;
  }

  // add the one been requested
  tree[theme] = {};

  // get inheritance
  var config = app.locals.themes[theme].zealderConfig;
  if (config && config.themeDependency && !tree[config.themeDependency]) {
    tree = getInheritTheme(config.themeDependency, tree);
  }

  return tree;
}
function getInheritTheme(theme, tree = {}) {
  if (!app.locals.themes[theme]) {
    // this theme is not installed
    app.error('requested theme:%s but it is not installed', theme);
    return tree;
  }

  // add the one been requested
  tree[theme] = {};

  // get inheritance
  var config = app.locals.themes[theme].zealderConfig;
  if (config && config.themeDependency && !tree[config.themeDependency]) {
    tree = getInheritTheme(config.themeDependency, tree);
  }

  return tree;
}
get theme inheritance