...
http://[GatewayServer]:[GatewayPort]/sap/bc/ui5_ui5/iqx/fab2/index_latest.html?sap-ui-appcache=false&interactive2=true&showlogoff=true&sap-ui-theme=custom_platinum@http://[GatewayServer]:[GatewayPort]/sap/public/bc/themes/~client-[ClientNumber]#StartForm/[ApplicationName]/_newForm
Alternatively, you can apply a Theme programmatically in your app using the following line of JavaScript code:
sap.ui.getCore().applyTheme("sap_fiori_3");
For a list of SAP themes supported by your current SAP UI5 version, please refer to this page:
Supported Combinations of Themes and Libraries