Can this font in the Preferences GUI be changed?
Not currently, but we can probably add it.
From the size of everything else, though, it looks like you're running Opus on a high DPI screen but have the system configured to standard DPI, and are adjusting the font sizes to compensate?
(Or the system it's configured to tell Opus it's in standard DPI, via Compatibility Settings, which should all be turned off. See Windows Compatibility Settings / Wrong DPI / Installer misdetects OS )
If so, adjusting the system DPI rather than the fonts in Opus is usually best. (Restart Opus after changing system DPI, i.e. File > Exit Directory Opus.)
Actually, you can already change it. At least here, just changing the Dialog font is enough:
Checked with Win10 and Win11.
Thanks for the info. My monitor is set at the recommended 2560 x 1440 display resolution with scaling at 150%. Compatibility items were all unchecked as you recommended. Make Text Bigger was at 120% and I returned it to 100%. The Preferences categories text seems the same size as before. That text seems to be unchanged when changing the Dialog font size.
I have scaled 125% in Windows and when I changed to font size 15 and restarted Dopus the headings followed and became the same size. Weird if it doesn't for you.
Offtopic to Leo: Where did you get your Russel Square font from, there are lots of different versions on the internet.
Not sure what installed it.
Scaled at 150% in Windows, I can't change Segoe UI to font size 15. It reports that it "may make Preferences too large for your screen." When I accept it anyway and exit and reenter DO, it forces a reset back to Segoe UI font 9. I selected Segoe UI font12 which DO accepts. The result is that, e.g., Colors and Fonts does not change size but items under it like Dark Mode do get larger.
That warning is there for a reason. The size you've chosen would make the dialog too large for the OK and Cancel buttons (and other elements) to even fit on your screen.
You can't use a font that large and still have things work. Choose a smaller size.