Hi @tim.sonin,
Happy to help.
You can add custom CSS classes to specific embedded forms by defining the cssClass attribute to the embed code, as outlined here.
If you added cssClass: 'ppc' to the desired forms’ embed codes, you could select the labels in these forms with the descendant selector:
.hs-form.ppc label {
color: #FFFFFF;
font-weight: 400;
}