The get_main_window was removed, probably in favor of using methods like GimpUi.window_set_transient(window) to attach/associate new windows to GIMP, but that is not required for your dialog box to match GIMP's theme.
What you are looking for is the method GimpUi.init("your_plugin_name"). Use it once to initialize GIMP's theming, then create your UI dialog. That method will make any new UI element match GIMP's theme.
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