How to change background color of M5Msgbox

[ondrej1024]

The documentation of the M5 LVGL API does not state a method for specifying the background color of a M5Msgbox.
However, I found that a M5Msgbox object has a style property which has a set_bg_color method . So I tried this:

myMsgboxObj.style.set_bg_color(lvgl.STATE.DEFAULT, lvgl.color_hex(0x888888))

This does not produce an error but meither does it change the backgound color of the box.

Any ideas?