i3wm: change QT/GTK theme

If you'd like to change your window appearance, you can use systemsettings utility for QT windows and .gtkrc conf for GTK windows.

systemsettings is part of the KDE (included in kdebase-workspace package). Navigate to Application Appearance to change window theme:

To change theme for gtk windows, edit/create file ~/.gtkrc-2.0:

include "/usr/share/themes/oxygen-gtk/gtk-2.0/gtkrc"
style "user-font" 
{
        font_name="Sans Serif "
}
widget_class "*" style "user-font"
gtk-font-name="Sans Serif  9"
gtk-theme-name="oxygen-gtk"
gtk-icon-theme-name="oxygen"
gtk-fallback-icon-theme="oxygen"
gtk-toolbar-style=GTK_TOOLBAR_ICONS
gtk-menu-images=0
gtk-button-images=0

or you can install kde-gtk-config (archlinux) and you should be able to edit these settings from systemsettings > Application Appearance > GTK: