I have installed unity in my Ubuntu Gnome installation and using Unity Tweak Tool to customize.
What is the default window title font in Unity?
(I have changed everything else)
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Below is a screen of "Ubuntu tweak" tool for font space settings.
Also, you may use below command to reset fonts back to its default.
gconftool-2 --unset /desktop/gnome/interface/document_font_name gconftool-2 --unset /desktop/gnome/interface/font_name gconftool-2 --unset /desktop/gnome/interface/monospace_font_name
Command Source: Default Window Title Font
Hope it helps!