How to query the GTK3 theme currently in use on Linux, enable dark mode and display missing icons in Gnucash

To view the current GTK3 theme in use on the XFCE desktop environment that is pre-installed in distributions like Linux Mint or Xubuntu, open a terminal window and enter the following comment:

$ gtk-query-settings theme <enter>

Upon enter, it displays the following:

gtk-theme-name: "Mint-Y-Dark-Orange"
gtk-icon-theme-name: "Mint-Y-Dark-Orange"
! gtk-fallback-icon-theme: NULL
gtk-key-theme-name: NULL
gtk-cursor-theme-name: "Bibata-Modern-Ice"
gtk-cursor-theme-size: 24
gtk-sound-theme-name: "LinuxMint"
gtk-application-prefer-dark-theme: FALSE

On my desktop, the icons for some buttons in GnuCash were not visible and using above, I found that the theme style I had selected was Mint-Y-Dark-Orange, but the icon-theme was Mint-Y-Orange. After I corrected the icon-theme in Settings->Appearance-Icons to Mint-Y-Dark-Orange, the icons in GnuCash displayed correctly.

Additionally, to apply the dark theme on the desktop to GnuCash registers, in Gnucash, go to Edit -> Preferences -> Register -> Graphics and uncheck the option Use GnuCash built-in color theme. To re-enable the built-in color theme, recheck the box.