Did you know that the current release of DecSoft App Builder includes up to 36 app's themes ready to be used by your developed apps? This means that you can add (in designtime) one or more app's themes in your apps, and, then you can change the app's theme in runtime, that is, by user demand, for example. Then only thing that you need to do is to include the themes that you want to be available, and, then use the "SetAppTheme" action, to set the app's theme that you wanted.
The app's themes of DecSoft App Builder are based in the well known and widely used Boostrap CSS framework, and, if yoy wanted, it's quite easy to add new themes to be ready for use in DecSoft App Builder and ready to be included in your developed apps. The only thing that we need to do is to copy a new CSS file (the new theme CSS file) in the App Builder configuration folder, under the Themes subdirectory. After that you can pick that theme to be included in your apps and to be used by default or just be ready to use in runtime.
Take a look at the below video: what you can see is the Themes app samples running. Please, take a look at the "Themes" and "Themes2" app samples, which are included in the installation of DecSoft App Builder. You can learn how to include, list and change the app's theme in runtime.