Tizen Developers

Hi,
No, I did not face such api. Looks like no simple way to set background. Only predefined styles could be set:
https://developer.tizen.org/dev-guide/2.3.1/org.tizen.guides/html/native/ui/styles_mn.htm
or here is example https://developer.tizen.org/dev-guide/2.3.1/org.tizen.guides/html/native/ui/component_button_mn.htm
Alexey.

To customize button you should perform something like this:
https://developer.tizen.org/dev-guide/2.3.1/org.tizen.sampledescriptions/html/mobile_n/ui_theme_extension_sd_mn.h
But this is quite time-consuming process. You should take source of button and redefine it fully in some edc file. It will be your "customized" theme which could be set with  elm_object_style_set(btn,"customized");
also you will need to add your new custom style name with
elm_theme_extension_add
Alexey