It seems the reset button only has effect in the current instance of the Applications Dialog.
It is possible to force LibreOffice to regenerate you user settings.
Regenarate you configuration.
WARNING: THIS WILL REPLACE ALL SETTING AND USER DATA.
This is for Windows.
Close any instances of LibreOffice.
registrymodifications.xcu file and rename it to something like
Restart LibreOffice and it will generate a new
registrymodifications.xcu file. Now all the defaults are back. Be warned all your history and user setting will be gone, well still in the
registrymodifications.xcu.bak file if you need to revert.
Note: IIRC in Linux the
registrymodifications.xcu is usually in
For Future endeavors make a copy of the theme you want to adjust by clicking save button in the dialog.
Now you can safely use and edit the new saved theme colors.