When that has happened to me, it was one of two things. Either the theme was not installed in the proper folder, or the folder where the theme is located, does not have proper permission set, to allow Tweaks access to it.
I suppose you could also install the wrong GTK version for the system too that you are using, but without us seeing what theme you installed, that one will be hard to determine.
Customize Best Zorin OS Themes Try what's been shown in this link. I can't recommend that much since I'm into minimalist style which I know not many people preferred so.
Have you extracted them in that way, that in your subfolder the themes-file is? Sometimes there are different themes packed into more subfolders, so you have to extract only a subfolder.
Just like mine. I don't have permission to the .themes to paste those themes. Hahahaah
Did you put them into home/.local/share/themes?
I'm going to give you a freebie cause I'm nice.
CTRL ALT T to enter terminal.
sudo nautilus
Once opened to root, navigate to your themes folder.
Now you have permission.
I'm going to bed. So your welcome and goodnight 
lol
yeah because I dont have .themes hidden (like in others) in my home directory. I even create a .themes folder in home or user directory since according to xfce manual, there should be that one. Just trying if it will work but it seems I can't easily fool my system.
I'm just trying out things and how far I can get to this. Thanks and good night
You can place themes in ~/.themes or in ~/.local/share/themes or in /usr/share/themes
If placed in ~/.themes or in ~/.local/share/themes, this is in your Home Directory and you should have permissions.
If not, something is wrong on your system.
Its already fine. Just like I said permission. Maybe because its encrypted? Hahahaha. That's what I only think the reason why for now. StarTrecker solution help me. I also see in reddit about cp -a for copying files through terminal as how it is "if" you don't have permission.
Also, the original poster who have problem here wants a full mac theme. I found one, https://www.xfce-look.org/ . It's not only for xfce. There's variety in that site so. I just try. Not a big deal to me. Learn a lot just by searching in this theme thing. hahaahha
No, it didn't.
Your home folder should not have directories within with Root permissions. This is not fixed, just bypassed.
Please open a Root Instance of the File Manager, just as StarTrekers suggestion above, but this time, navigate to root, then /home/ then Right Click that directory with your User name (Your home directory), go to Properties then permissions tab. Set the ownership to your username and file permissions for Owner and for Group, ensuring you click "Apply to enclosed files" at the bottom.
Thanks. I see that my access was only to read files but cannot create. I change that one.
Or you can just run:
sudo -i
Replace $USER with your username:
chmod -R a+rwX,o-w /home/$USER
did you already try to use it, copy paste in the themes folder just like what is suggested above?
i set download and extract the files in .themes folder in home file permission settings are:-
owner : me
permission:- create and delete files
group:- me
permission:- create and delete file
i installed two three themes before but now it isnt happening.
this can happens please check why i am not able to install this https://www.gnome-look.org/p/1572905
well!! that is my settings right now. but i want a better looking experience
you put it in the .themes in the home. Its default there? (cause to me there is no .themes there. There might be cases like that. If it was default there, and you have permission to create, did you copy everything after you extract from the archive?
files are as you said i dont know what is the problem
Then try placing it in the .themes in the root just like what I did. Follow the conversation from this: Full mac like theme or any other beautiful theme - #15 by StarTreker
by the way, the place there would be filesystem root/usr/share/themes you also need to place something in your icon folder
also, there is place for that panel. Plank folder.