As far as I can tell, it is Thorium that is behaving identical to Ungoogled Chromium v114 as far as fonts on Digital Spy.
But I'm unsure if they are doing what they are being "told" to do (at least as far as the first issue, you seem to have two issues here).
In any programming language, there is always a degree of "junk in, junk out".
You have two things going on.
First, your style sheet is MISSING the closing }.
Your style sheet should read -
from body {font-family: unset !important;
to body {font-family: unset !important;} (note that the only difference is the } at the very end.
Thorium and Ungoogled v114 both seem to add this missing } "for you".
Second, Digital Spy &/or Stylus is not applying your style sheet for "body" to "body h4" (unsure what the official CSS guideline is on this).
In your style sheet for Digital Spy, change it -
from body {font-family: unset !important;}
to body, body * {font-family: unset !important;} (note the added , body * so we catch not only "body" but "body h4" and "body h2", both being used at Digital Spy)