Hi Rafael,
you could try
filter: brightness(1.5)to make the gray icons whiter via CSS.
You might need to add -webkit-filter for support on some browser, and it won't work on IE11, but might be handy in some situations.
Hello Dave, Hello Mary, Hello David and other guys 
With your help and the great suggestion of David i solved this question.
I put in the theme_clean |customcss (http://your_ip_address/moodle/admin/settings.php?section=themesettingclean):
/* Top menu - Icon colors - White - Notifications and Messages */
[alt=" Toggle notifications menu "] { filter:brightness(1.5)!important; }
[alt=" Toggle messages menu "] { filter:brightness(1.5)!important; }
This solve the problem with the replace of the image in all the other places in the system. Very nice!
Thank you guys!
Tics
With your help and the great suggestion of David i solved this question.
I put in the theme_clean |customcss (http://your_ip_address/moodle/admin/settings.php?section=themesettingclean):
/* Top menu - Icon colors - White - Notifications and Messages */
[alt=" Toggle notifications menu "] { filter:brightness(1.5)!important; }
[alt=" Toggle messages menu "] { filter:brightness(1.5)!important; }
This solve the problem with the replace of the image in all the other places in the system. Very nice!
Thank you guys!
Tics
Excellent!
