UIFe: main action color on action windows

[Didier Roche]

Dear documentation team,

I want to request an UIFe for 
https://bugs.launchpad.net/ubuntu/+source/ubuntu-themes/+bug/1709500. 
Basically, the default action buttons aren't visible in our theme since 
most of applications are now using headbar. Consequently, a bunch of 
users may look for an "Ok" or "Save" button without realizing they are 
not at the bottom, but at te top.

We want to have something similar than in Adwaita, where the suggested 
default button is colored to make that obvious.

The used color is the ubuntu touch "green" action from our color 
palette, you will a screenshot attached on the bug.
Cheers,
Didier