First of all I had no problem with Libreoffice 5.4.x using high contrast on my Windows.
I use Windows 7.
The problem is that I updated to Libreoffice 6.2.4 (latest when writing this question) and seems like LO is not behaving properly with the black high contrast setting on my Windows
On windows 7, toolbar icons’ images are quite dark grey and background is black so the contrast is really bad.
On Windows 10 (at least mine) the interface and the document itself is all white text over white background, including the menus so can see nothing.
I noticed that LibreOffice has the option to detect the systems’ colors.
I had to go back to LibreOffice 5.4 because of this which is a shame.
I think this might be a bug. I don’t understand why in previous versions I had really no problem and buttons and all itnerface was white over black background, and now everything has gone really bad in terms of contrast.
I hope someone in the developer team notices this.