[Mint-X/Gtk3] Some issues and some possible issues

[Odyseus]

Hello, everybody.

@JosephMcc: I opened this issue instead of submitting a PR because some of the possible issues that I found might not be issues at all and you are a lot more acquainted with the Gtk3 changes.

• The scrollbar.overlay-indicator:not(.dragging):not(.hovering) button selector in gtk-widgets.css#L1962 has the background-color property duplicated.
• gtk-widgets-borders.css#L75 .button.default selector should (?) be button.default.
  • I saw that the shutdown dialog didn't have the Shutdown button highlighted with a border colored with the accent color. Changing the button class to the button tag fixed it.
• I saw several other places were the button class is used and I think (and I stress I think) that it should also be changed to the button tag.
  • The .mate-panel-menu-bar .button selector is used in several places (gtk-widgets-backgrounds.css#L12, gtk-widgets-backgrounds.css#L28 and gtk-widgets-borders.css#L11). But in gtk-widgets-borders.css#L29 the button tag is used instead of the class.
  • The .sidebar-button.button selector is used in several places (gtk-widgets-backgrounds.css#L21-L22 and gtk-widgets-borders.css#L26-L28). But in several places of the gtk-widgets.css file the button.sidebar-button selector is used.

[JosephMcc]

Cool. Thanks for taking the time to point these out. Mint-X is still a work in progress for gtk3.22. I've been pushing lots of changes lately. Some of those definitely look like things I missed. I'll look at gettingt them cleaned up. I've not updated the Mate support yet. That's one of the next on my list.

[JosephMcc]

Alright, cleaned up. Thanks again.
d8be299