Themeing

Dated, but here for posterity

Config file: ~/.config/gtk-X.Y/settings.ini

[Settings]
   gtk-theme-name=Equilux
   gtk-icon-theme-name=Ultra-Flat
   gtk-font-name=Sans 10
   gtk-cursor-theme-name=DMZ-Black
   gtk-cursor-theme-size=0
   gtk-toolbar-style=GTK_TOOLBAR_BOTH
   gtk-toolbar-icon-size=GTK_ICON_SIZE_LARGE_TOOLBAR
   gtk-button-images=1
   gtk-menu-images=1
   gtk-enable-event-sounds=1
   gtk-enable-input-feedback-sounds=1
   gtk-xft-antialias=1
   gtk-xft-hinting=1
   gtk-xft-hintstyle=hintfull
   gtk-xft-rgba=rgb

put the following in .profile to tell Qt programs to use gtk theme

export QT_QPA_PLATFORMTHEME=gtk2 # tested on Ubuntu 18.04

Calibre
Add to .profile

export CALIBRE_USE_SYSTEM_THEME=1