Appearance of applications: how to get wide character fonts on KDE?

[Guest]

Hi,
I have to help a person, using KDE 4.9.3, and I'm not an expert on KDE.
He has not a good sight, so I grew up the character fonts on the system (from the KDE configuration tool), so he is satisfied for the appearance of Dolphin or the desktop.

But what's about Gimp, LibreOffice or Firefox, the character fonts still undersized ! Is there a way to correct that ?

Thank you.

[HalationEffect]

It's because Gimp, LibreOffice and Firefox aren't native KDE applications. Rather than being built with the Qt toolkit that KDE programs use, they use the GTK+ toolkit instead.

If you look in System Settings -> Application Appearance -> GTK+ Appearance, you can set GTK+ applications to use the same fonts as your KDE applications.

If that doesn't solve the problem, it may be necessary to use the individual application's own settings to get the font sizes desired.