Last modified: 2014-04-02 00:56:48 UTC
I'm not sure, but I think this is a regression (it didn't use to be this way). A button that is flagged as constructive and currently disabled has a green border around its otherwise grey button. When hovering the button the border becomes grey, so it doesn't seem intentional that the border is green (otherwise it should have a hover implementation as well). This is most visible when opening VisualEditor, the save button has emits this behaviour.
Change 122879 had a related patch set uploaded by Krinkle: apex/button: Reset border-color of disabled button when not hovered https://gerrit.wikimedia.org/r/122879
Change 122879 merged by jenkins-bot: apex/button: Reset border-color of disabled button when not hovered https://gerrit.wikimedia.org/r/122879