The Firefox personas/themes in LO have been broken for a very long time.
There are a few optimistic sounding notes posted on Bugzilla, but as
noted you "/shouldn't expect this issue to be fixed in the foreseeable
future/".
If you have a recent version of LO installed, you will find 6 "personas"
in the LO installation directory C:\Program
Files\LibreOffice\share\gallery\personas\.
They appear in the LibreOffice -> Options -> Personalization as
"/Preinstalled themes/".
Here is a workaround I have used to overcome this LO defect:
* Pick one of the "/Preinstalled themes/" and apply it. For
illustration purposes only I am using "/Sand/".
* Shut LO down
* Take the .xpi you downloaded and unzip it to any convenient folder.
* Most likely the .xpi contains 3 graphic jpg or png files ( 1 - 200
px high, 1 -100 px high and preview image)
* Copy the 200px high and the 100px high files over to C:\Program
Files\LibreOffice\share\gallery\personas\sand
* If you get a message asking to overwrite, answer YES. Otherwise
delete the existing "Header.png" and "Footer.png" files and rename
the files extracted from the .xpi as "Header.png" and "Footer.png".
* If they are jpeg images rename them "Header.jpg" and "Footer.jpg"
and edit the 5th line of C:\Program
Files\LibreOffice\share\gallery\personas\personas_list.txt:
sand;Sand;sand/preview.png;sand/header.jpg;sand/footer.jpg;;#000000;#ffffff
* Restart LO and your theme should be displayed.
You say that you using a high contrast setting, in which case you might
need to tweak some of the 2 color settings in the C:\Program
Files\LibreOffice\share\gallery\personas\personas_list.txt file.
NOTE: Once you have a working configuration, make a backup copy of your
chosen theme (eg. sand), because it will not survive any LO update.
Hop this helps.
Dave