The stars are an image in the theme folder ... wp-content\themes\davinci\img\css
This is it's CSS
.stars .star {
background: url(./img/css/stars.png) no-repeat;
}
I suggest you either copy the star image, change it's colour (e.g with paint.net) and rename it to stars-2.png (or anything) or search Google images for another star with the colour you want and resize it if necessary. Then upload it to "wp-content\themes\davinci\img\css" via FTP
Make sure the image is a transparent background .png file.
Then paste this in Appearance/Customisation/Additional CSS
.stars .star {
background: url(./img/css/stars-2.png) no-repeat;
}
Don't forget to rename the star image to stars-2.png (or anything you want)
Because it has a different name, it won't be overwritten with theme updates.