The output in Forced Colors mode is different depending on if the light or dark mode is active

This is an interesting one. I do think it is important to give folks the choice of either light or dark mode. Glad that Gin does that. However, if someone has already specified their preference by specifying a high contrast mode, then there really isn't a need to support two versions of their preferences. Probably simpler to just support one consistently.

So putting up a warning here when forced-colors is active could be done by modifying the HTML:

<span class="fieldset__label fieldset__label--group">Appearance</span>

to

<span class="fieldset__label fieldset__label--group">Appearance <span class="forced-colors-warning" role="note">– Unavailable in Forced Colors Mode</span></span>

And adding this CSS:

/* Hide by default */
.forced-colors-warning {
  display: none;
  font-weight: bold;
  border: 0px solid transparent;
  color: ButtonText;
  background-color: ButtonFace;
}

/* Show only in forced colors mode */
@media (forced-colors: active) {
  .forced-colors-warning {
    display: inline;
    border-color: ButtonText;
  }
}