You can achieve that by adding the following code in the user command file.
Code:
icon_configs = [
# widget, named_icon, size
# auto mode buttons
("img_open", "open_file", 64),
("img_reload1", "refresh", 64),
("img_run", "play", 64),
("img_stop", "stop", 64),
("img_pause", "pause", 64),
("img_pause_active", "pause_active", 64),
("img_step", "step", 64),
("img_run_from", "run_from_line", 64),
("img_editor", "edit_code", 64),
("img_skip_optional_active", "skip_optional_active", 64),
("img_skip_optional_inactive", "skip_optional_inactive", 64),
# fullscreen
("img_fullsize_preview1_open", "fullscreen_open", 64),
("img_fullsize_preview1_close", "fullscreen_close", 64),
]
# default style context see TODO below
default_style = Gtk.Button().get_style_context()
failed_icons = 0
for widget_name, icon_name, size in icon_configs:
try:
image = self.widgets[widget_name]
pixbuf = icon_theme_helper.load_symbolic_from_icon_theme(self.icon_theme, icon_name, size, default_style)
image.set_from_pixbuf(pixbuf)
image.set_size_request(size, size)
except BaseException as err:
LOG.warning(f"Failed to change icon for <{widget_name}> to '{icon_name}': {str(err)}")
