The font colour is determined by your theme’s CSS stylesheet. To change it, you will need to either fix your theme’s CSS, or use the “Additional CSS” section when customizing your theme to add some custom CSS.
You can do something like this:
.glsr-shortcode {
color: #000 !important; /* Change the color as needed */
}
Please also see the following link which explains how to use the Google Chrome web inspector to determine the custom CSS changes to make: https://designtlc.com/use-chrome-inspector-edit-website-css/