The theme name passed as a parameter should be the value of the data-theme attribute used within the rel="dx-theme" links to the theme. For instance, if you have links to two of your themes,...
<link rel="dx-theme" data-theme="mytheme-dark" href="css/mytheme-dark.css" data-active="true" /> <link rel="dx-theme" data-theme="mytheme-light" href="css/mytheme-light.css" data-active="false" />
... you can switch between them in code in the following manner.
For details on the themes that are supplied with DevExtreme, refer to the Predefined Themes article.