I am using LibreOffice on Windows 7 since day one and previously used OpenOffice. I am super used to Galaxy icon theme and I can see how to set it back after installing LibreOffice 6.1 (up till today I have used LibreOffice 6.0.6).
In menu Tools | Options | LibreOffice | View | Icon style in LibreOffice 6.0 there is Galaxy theme, but it is missing in LibreOffice 6.1.
Looking into release notes: LibreOffice 6.1: Release Notes - The Document Foundation Wiki there is nothing written Galaxy theme was removed or something similar. This in my humble opinion is indication there got to be some special setting to enable Galaxy icon theme back.