So to get CSS to overwrite effectively, you just need to have the right level of specificity so as be more direct than competing styles. I am not entirely certain as to how the CSS Editor works for your specific theme, however, I assume that the generated CSS is simply not specific enough. Depending on what aspect of the Plugin you are trying to target, there are some selectors that are worth giving a shot.
For review display:
// all reviews are nested inside containers
.testimonial_group .testimonial {
}
// for specific targets you can use these
.testimonial_group .testimonial .rr_title {
}
.testimonial_group .testimonial .rr_date {
}
.testimonial_group .testimonial .stars {
}
.testimonial_group .testimonial .rr_review_text {
}
.testimonial_group .testimonial .rr_review_name {
}
For the form:
// All text inputs are of the form
.rr_review_form .form_table .rr_form_row .rr_form_input {
}
// And all labels are of the form
.rr_review_form .form_table .rr_form_row .rr_form_heading {
}
This should be more than enough to effect change on the font-size.
Let me know if you need further aid,
Thanks,
Charlie Maxwell
[NM_Developer]