LibreOffice Themes will replace the color customization
Quoting: LibreOffice Themes will replace the color customization - LibreOffice Design Team —
Since the first implementation of a dark color theme we continuously improved the customization of LibreOffice. In a GSoC projects this year, Sahil Gautam made it possible to not only change the application colors but also what is defined by the operating system respectively the desktop environment. This resulted in a major UI change for the upcoming version 25.2.
TL;DR: If you are using a dark theme with light application colors you have to load the extension Light Application Colors to restore the same look and feel in version 25.2.