The function ui_object_set_themeable_style_property() is not intended to be used like that, i.e. the can’t be overridden by calling the function again. (It will create a new colliding entry.)
If you really want to change the theme-colors in your LVGL code after exporting from SquareLine Studio, you should modify the style-variable’s value itself which was given as the 2nd argument of ui_object_set_themeable_style_property(). (In case of SquareLine export these are elements of const arrays which can be a problem in your case, but themes are usually not to be modified afterwards by coders). Then setting the theme again or calling _ui_theme_set_variable_styles() directly (or periodically) the change of the value will be sensed and the local style property (color) will be set.
(Yet seeing your use-case makes us consider adding a function to delete themeable style-properties, not just to add them.)
(I attached a projectfile which roundabout does what you describe above, just in case.)
ButtonColorThemeChanges.zip (10.1 KB)