It's correctly running in dark mode now. The problem was although dark mode was set on my standard user account, HWiNFO64 launches with admin permissions and my admin account had dark mode off (because of the fresh install that setting wasn't changed back).
Before I did a clean reinstall of Windows, HWiNFO would be in dark mode. Now, HWiNFO won't switch to dark mode.
I have 'dark' selected for 'choose your colour' in personalisation > colours (i.e. Windows is using dark mode for apps). I have 'default' selected for dark mode in general / user...