feat(org-chart): support external CSS themes and align documentation

examples/org-chart-react/host-theme.css

@@ -0,0 +1,7 @@
+/* Loaded by the host document. Colors inherit; Shadow selectors stay private. */
+.orgchart-custom-theme {
+  --hm-orgchart-company-hanmac: #2457a7;
+  --hm-orgchart-company-saman: #385ca8;
+  --hm-orgchart-expanded-line-color: #249a79;
+  --hm-orgchart-bg-surface: #edf4ff;
+}