My gtk3 theme is fine in the home directory but is completely borked in /usr/share/themes. Spent all day yesterday trying to figure out why and still have no clue.
Here's a comparison.
Theme in root
Theme in home
OK, here's a 'try-at-your-own-risk' solution:
Select 'file system' on the Left Panel in Nemo.
Navigate to /usr/share/
Scroll down until you come to the 'themes' folder -- then right click on it.
From the options that pop up, select 'Open as Root'.
The directory /usr/share/themes should then be open, with a red ribbon at the top of the screen saying 'Elevated Privileges'.
From that screen, go to the menu at the top and select 'Edit', then 'Select All'.
At that time all the folders in /usr/share/themes should be highlighted.
If all folders are highlighted, move your cursor into the panel with all the highlighted folders, and right-click your mouse.
Select 'Properties' at the bottom of the pop up menu.
'Properties' will give you a window that has two tabs at the top -- 'Basic' and 'Permissions'.
Select the 'Permissions' tab.
You will be presented with another window that looks like the one below.
There are three sections to that window -- 'Owner', 'Group', and 'Others'.
Under the 'Owner' section 'Folder Access' should be set to 'Create and delete files', and 'File Access' should be set to 'Read and write'.
Under the 'Group' section 'Folder Access' should be set to 'Access files', and 'File Access' should be set to 'Read only'.
Under the 'Other' section 'Folder Access' should be set to 'Access files', and 'File Access' should be set to 'Read only'.
Once those options are selected, there is a button at the bottom of the screen that says 'Apply Permissions to Enclosed Files'.
Hit that button. There won't be any big fanfare, or acknowledgement of you changes, but all folders and files with /usr/share/themes should be set to the correct permissions.
Click the 'X' button at the top right hand corner of Nemo. You should have to do this to close your 'Root' session and then your 'User' session.
Now, when you go to access your 'themes', you should have all of them available to you.
If you followed these steps correctly, and you still have a problem -- call the fire department
If you try this, please post back the results.
BTW, I know there is a way to do this by use of the terminal, but I never learned the commands to handle it that way