Can I change the layout at the question-level without overriding the entire layout?

MilBu
This is quite unusal and I'd probably tend towards modifying the css of specific elements of the questions you want to change, but you could try this at your own risks. :)
First, identify the link href of the theme in your first survey. Do the same for a second survey containing the theme you want to apply to your specific questions.
Apply this to your question's custom JS :
Qualtrics.SurveyEngine.addOnReady(function()
{
let newTheme = document.querySelector('link[href="/jfe/themes/templated-skins/SURVEY 1 THEME URL/stylesheet.css"]');

if (newTheme) {
  newTheme.setAttribute('href', '/jfe/themes/templated-skins/SURVEY 2 THEME URL/stylesheet.css')
}
});
Reapply the code but inverse the link href at the following question to come back to the original theme.
Be sure to test extensively, this might easily lead to unexpected behavior. :)