[This sounds like something that might be a Qt bug unless kdiff3 is doing something funky.]
I just installed kdiff3 0.9.98 (64 bit) on Windows 7 from the binaries in sourceforge. The about dialog says it uses Qt 5.3.0.
I have the free DejaVu fonts installed on the computer. Control Panel -> Appearance and Personalization -> Fonts -> DejaVu Sans Mono lists these fonts:
Now, I thought I would like to use DejaVu Sans Mono Book as the font, so I did the following in kdiff3:
Now, I would expect to find in the Font style dialog all the available styles for DejaVu Sans Mono. However, the only available options are Italic and Bold Italic. (There's obviously some remapping of names somewhere here, since it says Italic, not Oblique.) The same is true for e.g. DejaVu Sans and DejaVu Sans Condensed. For DejaVu Sans Light, the options are Light, Bold, Light Italic and Bold Italic.
Even more weirdly, while choosing DejaVu Sans Mono Italic gave me that italic font in kdiff3, after restarting kdiff3 it uses the DejaVu Sans Mono Book (which is exactly what I want). Still, the font dialog doesn't show that option...