Details:
If you are using a theme like Arc-Darker or Arc-Dark, software like Libreoffice will look messed up, especially the top bar is dark and doesn't fit with the rest of the things. It took me some time but I found a nice temporary fix for this.
First, this is how Libreoffice Writer look with Arc-Darker :

This is how it looks with Arc :

Now, if someone experiences this, he/she has to sudoedit the file at /usr/lib/libreoffice/program/soffice and insert this environment variable : GTK_THEME=Arc , like this :

Now, if you open Libreoffice Writer or Base, it will look fine. I'm glad I solved this for now. :) Other solutions I found were editing the file at /etc/profile.d/libreoffice.sh and uncommenting gtk, but that affects the fonts and other things.
EDIT : See my last post.
I have to comment because you sir are a f**king legend!
I opened a bug on libreoffice (https://bugs.launchpad.net/ubuntu/+source/libreoffice/+bug/1580537) a while ago and i have been frustrated by libreoffice since then.
Thank you!
Hmm, in my opinion forcing Libreoffice to use GTK2 by adding export SAL_USE_VCLPLUGIN=gtk to /etc/profile is a better solution
@horst3180
It depends. I had that uncommented for a while. It affects the fonts in some way, unpleasant. With gtk commented, I don't have that problem.
Edit : I made some screenshots.
SAL_USE_VCLPLUGIN=gtk uncommented

SAL_USE_VCLPLUGIN=gtk commented

I agree, the interface looks a bit better, it fits with the theme. But look at the fonts, they become bold, I don't want that.
This is not really a issues with Arc, the theme is fantastic. @horst3180 , your theme is very beautiful, one of the best for sure, I don't doubt. I want to close this now, maybe it will be helpful for other people if they search for this issue with LibreOffice. :)
I did something yesterday and found the perfect configuration for LibreOffice. I think I was a bit wrong above. As horst3180 said, uncommenting export SAL_USE_VCLPLUGIN=gtk in /etc/profile.d/libreoffice.sh is better but I had a problem : I was using Ubuntu Medium as my system font and the fonts in LibreOffice looked bad.
So I researched a bit, opened LibreOffice Writer, went to Tools > Options > Fonts > Check "Apply replacement table" and replaced the Ubuntu font with Arial. Now everything looks perfect. :)
I have the same issue using Arc-Flatabulous Dark theme on Ubuntu. How can I sudoedit the file?
Most helpful comment
I have to comment because you sir are a f**king legend!
I opened a bug on libreoffice (https://bugs.launchpad.net/ubuntu/+source/libreoffice/+bug/1580537) a while ago and i have been frustrated by libreoffice since then.
Thank you!