I found how to disable the plugin CSS, not how to avoid changing it every time I upgrade:
“Turn off CSS
Prevents loading of the mappress.css stylesheet. If you select this option you must copy all map styling to your own theme stylesheet, ‘styles.css’.”
The suggested workaround just forwards the problem to the theme, and is reset every time I upgrade the theme.
Best would be if this was left with default margin setting (why do we need the map indented by default?).
Second best would be if it was made available as options in the plugin’s settings panel.