I also tried this:
[alt=" Toggle notifications menu "] { background: white!important; }
And the background worked but the "fill" option on the svg didn't. I guess because i couldn't identify what the correct path for the img :/
Here is the link to the discussion where I found a solution for a similar request.
https://moodle.org/mod/forum/discuss.php?d=349402#p1409794
The alternative is find the SVG images and copy them from moodle/pix/i/messages.svg and moodle/pix/t/notifications.svg
Then place them in a folder in the root directory of your theme. Name this new folder pix_core
and add the images into their respective folders I and t
so so the list of directories is as follows:
moodle/
theme/
yourtheme/
pix_core/
i/ notifications.svg
t/ messages.svg
Here is the fun bit...
using Notepad open the svg files and find the part of the code that says
fill: #999999;
here is source code for the svg file
https://github.com/lazydaisy/moodle/commit/43f5b288c22be067d769bb7c0df501fb7ce70d0a
and change to
fill: #ffffff;
Save and then check to see if the images change color. if not then do a Purge all caches
If it doesn't work then the images that are being used are not svg they will be png or gif
hope it helps?
Mary
Hello Mary 
Thanks for the tips. In my case i cant change the SVG file directly cause it will turn the icon of notification or message in white on all the system so i have to do it by css only in the menu... unfortunately i didnt find the way to the svg image to use the fill attribute correctly :/
If i have succeed i'll post here
Cheers,
Tics
Thanks for the tips. In my case i cant change the SVG file directly cause it will turn the icon of notification or message in white on all the system so i have to do it by css only in the menu... unfortunately i didnt find the way to the svg image to use the fill attribute correctly :/
If i have succeed i'll post here
Cheers,
Tics
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!
