css.entry-content p,
.entry-content div,
.entry-content span,
.entry-content li {
  font-family: 'Spectral', serif !important;
  font-weight: 400 !important;
  font-size: 16px !important;
  line-height: 1.8 !important;
  letter-spacing: 0.02em !important;
}

.entry-content h1,
.entry-content h2,
.entry-content h3 {
  font-family: 'Spectral', serif !important;
  font-weight: 400 !important;
  letter-spacing: 0.08em !important;
  text-transform: uppercase !important;
}
The !important forces the font to override anything ProPhoto is setting on top of it.
Save it, then hard refresh your page — on Mac press Cmd + Shift + R, on Windows press Ctrl + Shift + R.
Does the font change after that?