Our host company came up with an easy fix which anyone can do without having to access the theme folder to delete and rename files etc as advised in the README.txt:
In the Custom CSS and JS add this bit of code:
.smallicon {
filter: invert(100%);
}
Hi, regards the dissapearing messages icon, you may find it is there but a black image which is not showing on the black bacground.
I recently swithed these so the current version of the theme should load white images, both are included in the theme, you have to rename the images to choose which you use (black or white).