Thanks! Shortening it to
font-family: 'Space Grotesk', Noto, Arial, sans-serif;
fixed the Report Viewer page.
Edit: Shortening it as far as you did, the Image Viewer still does not show Space Grotesk. Removing sans-serif causes other forms (e.g., custom log-in prompt) to revert to a serif font. But with it now down to one Form, I can probably manually override its labels.
Edit 2: Nope. Entering Space Grotesk as the font name just results in a serif font being displayed. Is there some special syntax for font names containing spaces? I tried entering a + instead of a space; single quotes; double quotes. Same result.