Hi Didier,
The stars are part of a sprite in gif format:
https://zenoweb.nl/wp-content/plugins/gwolle-gb-addon/assets/rateit/star.gif
You could edit this image in any image editor and change the colors. Then upload it to your website to its own url. Do not overwrite the image from the plugin, that will be overwritten again on an update of the plugin. The media library would even work, but better is a separate place with assets for your website.
With CSS you can create style rules to use your own image.
For example:
html body .gwolle-gb .rateit .rateit-preset {
background-image: url('https://zenoweb.nl/wp-content/uploads/2023/08/star-darker.gif');
}Does this help you? Can you manage?