Well I’ve tried editing /etc/environment, tried creating ~/config/environment.d/bash.conf, tried creating ~/.config/environment.d/libreoffice.conf, tried logging out then back in, tried a full reboot, nothing has worked so far.
your final objective is: what?
not a white sheet?
something else?
For libreoffice to follow the system theme when launched from thunar when opening a file. Why do people keep bringing up a white sheet? I’m not having issues with the document colours, just the UI colours.
Because people do not understand what your issue is.
I do not even understand it now, after all this.
Look at the screenshot, the dark UI is what I expect to get, the bright one is what I actually get.
I know. I see.
a white sheet and a dark/black sheet.
So:
WTF? 
No, not the sheet, that’s just a result of inverted system colours (not supposed to happen, same file). The UI, User Interface, ignore the document entirely.
I will be leaving this topic here and now.
I still have no idea what OP’s problem is nor do I care about it because I do not even use libreoffice …
so:
I better just shut up.
How can I be more clear? GUI? UX? What term makes you think of the only thing UI refers to? I even said to ignore the document entirely as it’s not relevant to the problem. I gave a screenshot of the problem. Do I need to make a video and point with the mouse to the upper section of the windows to highlight the broken UI theme? I honestly don’t understand how I can be more clear than the official term for those elements.
I don’t care.
I don’t have a problem.
I don’t have the problem that you perceive to be one.
If you have a problem - you need to convey it to others in a way they can relate to and understand.
That’s it, really.