Remove image border

I recommend asking at https://wordpress.org/support/theme/astra so the theme’s developers and support community can help you with this.

Add to the Additional CSS panel of the customizer:

.gallery-caption {
    border: 0 solid #fff;
}

BTW, this isn’t a Developing WordPress topic, I’m moving it to Fixing WordPress. Don’t try to make sense of our organizational logic, we move topics around all the time.