As @DK Bose stated select: Tools > Options > LibreOffice > View > Icon Style
Then select the icon theme as the image shows bellow:
For those that the user interface / system language is in Greek should select Εργαλεία > Επιλογές > Προβολή
and then maximize or enlarge the window (the options not shown correctly for some reason in the norma sized window) and then select the options as shows the image bellow:
The Oxygen
icon theme gave me colorfull icons that I can see them even on a dark theme.
Also in case that you use neither Greek nor English as system's language then you can launch the libreoffice in English
by typing at the terminal:
LANG=C libreoffice --writer
Then follow the steps mentioned for English
environment.
Tools > Options > LibreOffice > View > Icon Style
and see if you like something there.