To open the Theme Customizer, click the paintbrush icon in the left menu bar. There is no left menu bar, and no option to get it (or make it visible).
Can you maybe give more info? You opened your app in Studio and not studio pro. Then there is no paint brush in the left menu?
Rick van Huffel
The theme customizer option is in Mendix Studio, can you please confirm if you are using Mendix Studio and still facing the issue?