so try disabling the Merged Site Header for mobile. Go to the Header Element > Site Header Tab and on the merge settings you can set it to desktop only. You Mobile header will then use the colors you have set in the customizer.
Aah actually you’re using the Legacy Page Headers, in that case try this CSS:
.main-navigation.toggled .main-nav > ul {
background-color: #3f3f3f;
}
If you want to check out the new Elements modile in the future then you can learn more here, but if the old page headers are working for you then there is no need to change: