Custom GTK Themes always have menu bars with no padding

Question

I'm having a consistent problem when trying to use custom GTK themes in Ubuntu, as well as Linux Mint.

For every theme that I try, the padding on the menu buttons is gone, except for in the default theme, and there are areas where widgets are "too close together".

Here are examples of what I mean:

Ubuntu with the United-Ubuntu theme:

Linux Mint, also with the United-Ubuntu theme, same problem:

Ubuntu with the Vimix theme:

I usually just search for an answer from the internet for all of my Linux problems, but this is a special case where I cannot find anything on this whatsoever - I seem to be the only person with this sort of problem.

I'm guessing it's not a consistent design choice - is there something wrong with my setup? Do I need to update my software? Will I need to edit the .css files myself?

Any general steps in the right direction will be highly appreciated - it would make finding and using custom themes a treat instead of a chore!

Ubuntu Version: 16.04 LTS

Linux Mint Version: 18.3 Cinnamon

Both of the above screenshots were taken after installing, and after running sudo apt-get update and sudo apt-get upgrade.

Answer 1

Simple answer is those themes are incompatible with your system. Both the themes require GTK+ 3.20 or later, whereas Ubuntu 16.04 LTS has GTK+ 3.18 (or some lower version, you can check it by running apt-cache policy libgtk-3-0). So naturally those themes don't work properly in your system.

You can try requesting the developers of these themes to provide support for GTK+ 3.18 and hope that they'll do so.

You may also consider upgrading to a newer release like Ubuntu 17.10 or Ubuntu 18.04 LTS, both have GTK+ 3.22.