Hi Adrian,
I decided to install open office to see what you mean.. The blue buttons with black text is very hard to read. Worse if you have increased Contrast selected in the Accessibility PreferencePane.
After playing with different settings the only thing that I found was to change the buttons to Gray with Black text. Still not super easy to read, but better than the blue text.
To change the buttons to gray, you need to do this system Prefs. Go to General, and then under Accent Color select the Gray which is the far right option. Selecting any other color does not change the Blue background of the button
This was done i macOS Big Sur, so the location of the accent color may be different f you have a newer version of macOS.