With UI5/openui5@01e9de7 UI5 offers a fallback and default theme mechanism.
Therefore, the best practice is to no longer configure the desired theme in the UI5 boot configuration but let UI5 or an including service (e.g. sap.ushell) decide about the theme to load.

Outdated usage

<script id="sap-ui-bootstrap"
    type="text/javascript"
    src="resources/sap-ui-core.js"
    data-sap-ui-theme="sap_horizon">
</script>

Recommended usage

<script id="sap-ui-bootstrap"
    type="text/javascript"
    src="resources/sap-ui-core.js">
</script>