Hi Alain and welcome,
This may be a good opportunity for you to start learning to mod a little. The graphics for the Nav ball are located in the following Kerball directory:
\KSP\GameData\Squad\Props\IVANavBall
The graphics are *.dds files so you will need a program to edit them. I use Gimp: https://www.gimp.org/downloads/
After you install Gimp, you will need to install the dds plugin. This plugin can be found on gimps website: http://registry.gimp.org/node/70
Drop this plugin inside the following folder of Gimp: C:\Program Files\GIMP-2.0\lib\gimp\2.0\plug-ins
Now you can use Gimp to open those *.DDS files.
-K
I forgot to mention,
You will also need to save the file the default extension Gimp wants it saved as, and then reopen it and export it as a *.dds file.
And
That dir is for the nav ball inside the craft. if you want to change the main (exterior) nav ball, edit the files in:
\KSP\GameData\Squad\Props\NavBall
If you contrast the nav ball a little, it will really make those Nav Markers pop.
-K